Trends of Yield and Soil Fertility in a Long-Term Wheat-Maize System

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1 Journal of Integrative Agriculture 214, 13(2): February 214 RESEARCH ARTICLE Trends of Yield and Soil Fertility in a Long-Term Wheat-Maize System YANG Xue-yun 1, 2, SUN Ben-hua 2 and ZHANG Shu-lan 1, 2 1 State Key Laboratory of Soil Erosion and Dryland Farming, Northwest A&F University, Yangling 7121, P.R.China 2 Key Laboratory of Plant Nutrition and the Agri-Environment in Northwest China, Ministry of Agriculture/Northwest A&F University, Yangling 7121, P.R.China Abstract The sustainability of the wheat-maize rotation is important to China s food security. Intensive cropping without recycling crop residues or other organic inputs results in the loss of soil organic matter (SOM) and nutrients, and is assumed to be nonsustainable. We evaluated the effects of nine different treatments on yields, nitrogen use efficiency, P and K balances, and soil fertility in a wheat-maize rotation system ( ) on silt clay loam in Shaanxi, China. The treatments involved the application of recommended dose of nitrogen (N), nitrogen and phosphorus (NP), nitrogen and potassium (NK), phosphorus and potassium (PK), combined NPK, wheat or maize straw (S) with NPK (SNPK), or dairy manure (M) with NPK (M1NPK and M2NPK), along with an un-treated control treatment (CK). The mean yields of wheat and maize ranged from 992 and kg ha -1 under CK to and kg ha -1 under M2NPK treatment, respectively. Treatments in which either N or P was omitted (N, NK and PK) gave significantly lower crop yields than those in which both were applied. The crop yields obtained under NP, NPK and SNPK treatments were statistically identical, as were those obtained under SNPK and MNPK. However, M2NPK gave a significant higher wheat yield than NP, and MNPK gave significant higher maize yield than both NP and NPK. Wheat yields increased significantly (by 86 to 155 kg ha -1 yr -1 ) in treatments where NP was applied, but maize yields did not. In general, the nitrogen use efficiency of wheat was the highest under the NP and NPK treatments; for maize, it was the highest under MNPK treatment. The P balance was highly positive under MNPK treatment, increasing by 136 to 213 kg ha -1 annually. While the K balance was negative in most treatments, ranging from 31 to 217 kg ha -1 yr -1, levels of soil available K remained unchanged or increased over the 2 yr. SOM levels increased significantly in all treatments. Overall, the results indicated that combinations of organic manure and inorganic nitrogen, or returning straw with NP is likely to improve soil fertility, increasing the yields achievable with wheat-maize system in a way which is environmentally and agronomically beneficial on the tested soil. Key words: wheat, maize, nitrogen use efficiency, P balance, K balance, soil organic matter INTRODUCTION Sustainable increase in crop yields is needed to ensure food security in China, the world s most highly populated country. Careful scientific management of soil will be essential if this demand for cereal production is to be met. Soil management practices have profound impacts on soil organic matter (SOM) level, which is closely linked to land productivity (Zhou Received 31 October, 212 Accepted 7 April, 213 YANG Xue-yun, Tel: , xueyunyang@nwsuaf.edu.cn, xueyunyang@hotmail.com; Correspondence ZHANG Shu-lan, Tel/Fax: , zhangshulan@nwsuaf.edu.cn, zhshulan@hotmail.com doi: 1.116/S (13)6425-6

2 Trends of Yield and Soil Fertility in a Long-Term Wheat-Maize System 43 et al. 213). Organic amendments play an important role in the improvement of SOM levels and thus soilphysical properties (Hati et al. 27). However, in recent years, farmers in Asia have increased their usage of inorganic fertilizers at the expense of organic materials due to increased labor costs and the advent of mechanized harvesting. For example, in 1986, organic manure was applied to rice fields in the Yangtze Delta at an average rate of around 5 kg N ha -1, accounting for 3% of the total N applied in the year; by 1997, the average had fallen to 2 kg N ha -1, making up only 6.7% of the total N applied (Zhu and Chen 22). Crop residues are commonly burned in the field in order to keep them from hindering tillage and seeding operations for the next crop. However, disposal of crop residues in this fashion reduces the quantity of organic carbon which is returned to the soil; moreover, the practice causes atmospheric pollution and has been blamed for declining levels of soil nutrients (Nguyen et al. 1994). The recycling of organic resources is becoming a progressively significant aspect of sustainable agriculture for its role in minimizing environmental problems of this kind (Pathak et al. 26). Yadav et al. (2) and Surekha et al. (26) found that organic amendment had positive effects on rice and wheat yields in the Indo-Gangetic Plain region, although contradictory results were observed by Dawe et al. (23) in their studies on intensive rice cropping systems. This apparent discrepancy may be due to differences in soil type and the quality of the organic materials applied in the different studies (Dawe et al. 23; Chivenge et al. 211). Winter wheat-summer maize rotation is the principal agricultural production system used in central and northern China, around 16 million ha are cultivated in this fashion, comprising about one quarter of the total national food production. Since the late 197s, rapid increase in the use of chemical fertilizers and the introduction of high-yielding varieties have resulted in substantial increases in yield per ha. However, residual effects, soil degradation and poor land management, as well as other social, economic and technical issues have caused similarly dramatic declines in crop yield per unit of chemical fertilizer applied (Tong et al. 23). For example, the partial factor productivity of nitrogen in the intensive wheat-maize system has decreased from 46 kg kg -1 at an application rate of 174 kg N ha -1 yr -1 in 1978 to 21 kg kg -1 at an application rate of 592 kg N ha -1 yr -1 in 1998 (Fang et al. 26). This has caused a significant decrease in fertilizer use efficiency (N, P and K) (Zhang et al. 28). Meanwhile, the increasing use of chemical fertilizers is contributing to air and water pollution in China (Zhu and Chen 22). Long-term experiments are powerful tools for research on the sustainability of management practices and can be used as a sort of early warning system to identify problems that may threaten future productivity (Berzsenyi et al. 2). In the study reported herein, we analyzed results from a long-term experiment conducted over a 2-yr period at the Chinese National Soil Fertility and Fertilizer Efficiency Monitoring Base of Loessial Soil, Yangling, Shaanxi, China. The work presented here is intended (i) to examine the yield trends of winter wheat and summer maize under longterm amendment of organic and mineral fertilizers; (ii) to investigate dynamics of SOM, P and K nutrients in plough layer soils under the intensive wheat-maize rotation cropping system; and (iii) to analyze N use efficiencies, and estimate the apparent balances of P and K of the system at a long run. RESULTS Mean yield The mean grain yields of wheat obtained over the 2-yr experiment showed that the M2NPK treatment resulted in a significantly higher yield than the NP, PK, NK, N, and CK treatments, but had a similar effect to treatments receiving M1NPK, SNPK and NPK (Table 1). The yield obtained with the NP treatment was identical to those with M1NPK, SNPK and NPK, but significantly greater than those with PK, NK, N, and CK (all of which gave similar yields to one another). Maize yields did not differ significantly between treatments involving the incorporation of organic materials (i.e., MNPK and SNPK), but the yields for the MNPK treatments were substantially higher than those treatments receiving chemical fertilizers alone. However, there were no significant differences between the yields of the SNPK and NPK or NP treatments (Table 1). In addition, the NPK and NP treatments gave significantly

3 44 YANG Xue-yun et al. higher maize grain yields than the PK, NK, N, and CK treatments did. In sharp contrast to wheat, the maize grain yield was detectably higher for the NK treatment than for N, both of which gave substantially greater maize yields than PK or CK (Table 1). Table 1 Average yields of wheat and maize in the long-term wheat-maize experiment ( ) Treatment Wheat (kg ha -1 ) Maize (kg ha -1 ) CK 992 (379) c (514) e N 13 (592) c 3 2 (777) d NK 1269 (525) c (813) c PK 1248 (44) c (869) e NP 5329 (1168) b (189) b NPK 5476 (1244) ab (162) b SNPK 5573 (129) ab (194) ab M1NPK 5668 (1519) ab (1187) a M2NPK 5962 (12) a (1454) a Numbers in parentheses are standard deviations. Values within each column followed by the same letter did not differ significantly (P<.5) as judged by the LSD test. The same as below. Yield trends It was observed that the yields of wheat declined over time in the CK, N and NK treatments; the decline was especially pronounced in the N treatment (Fig. 1). By contrast, wheat yields in plots that received organic amendments (i.e., plots subjected to the SNPK and MNPK treatments) exhibited significant positive trends with the average yields increasing from kg ha -1 yr -1 in M2NPK to kg ha -1 yr -1 in SNPK (Fig. 1). Treatment with NP or NPK fertilizers resulted in significant year-on-year increases in wheat yield (P<.1 and P<.5, respectively). In contrast to the yield trends of the winter wheat, maize yields did not change significantly over the years on any treatment other than on PK, where a pronounced year-on-year increase was observed (Fig. 2). Nitrogen use efficiency The apparent nitrogen use efficiencies varied markedly (Table 2). The mean nitrogen recovery efficiency (RE) of winter wheat ranged from about 12 to 66%. The highest values were observed on treatments of NP and NPK, followed by SNPK and MNPK and then N CK N NK y= x r=-.296 y= x r=-.532 * y= x r=-.218 Wheat grain (kg ha -1 ) PK y= x r=.126 NP y= x r=.436 NPK y= x r=.559 * SNPK 1 M1NPK M2NPK y= x r=.614 ** y= x r=.62 ** y= x r=.68 ** Year Fig. 1 Grain yield trends for winter wheat under diverse fertilization regimes over 2 yr. * and ** denote significance at.5 and.1 levels, respectively. The same as below.

4 Trends of Yield and Soil Fertility in a Long-Term Wheat-Maize System 45 1 CK y = x r=.26 1 N y = x r= NK y = x r=-.29 Maize grain (kg ha -1 ) 1 PK y= x r=.54* 2 1 NP y = x r=.79 1 NPK y= x r= SNPK 1 M1NPK 1 M2NPK y= x r=.28 y= x r=.182 y= x r=.254 Year Fig. 2 Grain yield trends for summer maize under diverse fertilization regimes over 2 yr. and NK treatments. The mean agronomic efficiency (AE) for winter wheat was in the range of about 1-28 kg kg -1, with the highest levels being achieved on treatments of M1NPK, NPK and NP, and the lowest on N and NK. M2NPK and SNPK gave significantly lower AE values than the M1NPK, NKP and NP did. The partial factor productivity of nitrogen for winter wheat varied from 6.2 to 34.4 kg kg -1, and responded to the different treatments in the same way as AE (Table 2). The mean RE for maize ranged from ca. 18 to 51% (Table 2), the highest value was observed on MNPK plots, with the SNPK, NP and NPK, NK, and finally N treatments yielding successively lower values. The lowest AE value observed was 4.7 kg kg -1 on plot treated with N alone and the highest was on M2NPK (Table 2). The PFP for maize ranged from 16 kg kg -1 on N treatment to ca. 37 kg kg -1 on M2NPK, and exhibited similar trends to those observed for AE (Table 2). Apparent nutrient balance The apparent P balance at the system level was positive in all treatments except for those in which no P was applied (Table 3). Over the 2 yr experimental period, the average P balance ranged from 39 kg ha -1 yr -1 for the NP, NPK and SNPK treatments to 213 kg ha -1 yr -1 for the M2NPK treatment. In contrast, P level declined at a rate of approximately 1 kg P ha -1 annually on plots where no additional P was applied (i.e., those subjected to the CK, N, and NK treatments). Significant accumulation of P (69 kg P ha -1 annually) was observed on plot given PK fertilizers (Table 3). The apparent K balances were negative for all treatments save for M2NPK (in which large quantities of organic manure were incorporated annually) and treatments in which K fertilizer was applied but either N or P was not, i.e., NK and PK (Table 4). On average, the K balances varied from -217 kg ha -1 yr -1 under NP to 6 kg ha -1 yr -1 under PK. Soil fertility parameters Soil organic matter (SOM) SOM levels differed significantly between the various treatments (Table 5). The highest mean SOM content was observed under

5 46 YANG Xue-yun et al. Table 2 Nitrogen use efficiency (recovery and agronomic) and partial factor productivity of nitrogen for wheat and maize in a long-term wheat-maize experiment, Yangling, China ( ) Winter wheat Summer maize Treatment RE (%) AE (kg kg -1 ) PFP (kg kg -1 ) RE (%) AE (kg kg -1 ) PFP (kg kg -1 ) Mean Range Mean Range Mean Range Mean Range Mean Range Mean Range N 15. d c c f d d NK 12.1 d c c e c c NP 6.9 ab a a d b b NPK 65.7 a a a c b b SNPK 56.1 bc b b b ab ab M1NPK 54.6 c a a a a a M2NPK 53.8 c b b a a a RE, recovery efficiency; AE, agronomic efficiency; PFP, productivity of nitrogen. Table 3 Apparent P balance between 1991 and 21 in a long-term wheat-maize experiment, Yangling, China Treatment Mineral fertilizer Organic materials Input (kg ha -1 ) Output (crop uptake, kg ha -1 ) Rainfall Seed Total Wheat Maize Total Balance (input-output) (kg ha -1 ) CK N NK PK NP NPK SNPK M1NPK M2NPK Table 4 Apparent K balance between 1991 and 21 in a long-term wheat-maize experiment, Yangling, China Treatment Mineral fertilizer Organic materials Input (kg ha -1 ) Output (crop uptake, kg ha -1 ) Rainfall Seed Total Wheat Maize Total Balance (input-output, kg ha -1 ) CK N NK PK NP NPK SNPK M1NPK M2NPK M2NPK, followed by M1NPK and then SNPK. The application of NP or NPK also significantly increased SOM levels relative to the control treatment (CK). If either the N or the P fertilizer was omitted, as in the N, NK and PK treatments, the SOM levels were similar to those in the control. Besides SOM levels increased markedly over time in both the fertilized and the control (CK) plots (Fig. 3 and Table 5); annual increases ranged from.95 to.814 g kg -1. Available phosphorus (Olsen-P) The examined various nutrient management regimes had significant effects on the Olsen-P concentration (Fig. 3 and Table 5), the mean values of which ranged from 3.83 mg kg -1 under CK to mg kg -1 under M2NPK. Treatment with P fertilizer significantly increased the Olsen-P level. The incorporation of straw in addition to NPK treatment resulted in a further small increase in the availability of soil Olsen-P, while continuous treatment with manure in addition to NPK treatment over 2 yr resulted in a dramatic increase in soil Olsen-P; the Olsen-P level in the M2NPK plot exceeded 2 mg kg -1, with annual increases of up to 7 mg kg -1 yr -1 (Fig. 3). Available potassium The mean available soil K concentrations ranged from mg kg -1 in CK to mg kg -1 in M2NPK (Table 5). The addition of

6 Trends of Yield and Soil Fertility in a Long-Term Wheat-Maize System 47 Table 5 Average soil organic matter (SOM), available phosphorus (Olsen-P) and available potassium (AK) levels in the tested soil and their behavior over time in a long-term wheat-maize experiment in Yangling, China ( ) Treatment Mean (g kg -1 ) SOM Olsen P AK Change (g kg -1 yr -1 ) P value Mean (g kg -1 ) Change (mg kg -1 yr -1 ) P value Mean (mg kg -1 ) Change (mg kg -1 yr -1 ) CK e d e N e d e NK e.158 < d c 9.68 <.1 PK e.189 < c c 1.2 <.1 NP d.231 < cd e NPK d.291 < cd d SNPK c.385 < c c 9.78 <.1 M1NPK 2.73 b.597 < b b <.1 M2NPK 24.9 a.814 < a a 14.8 <.1 P value an inorganic K-containing fertilizer greatly increased the levels of available K in the soils, which could be further increased by the incorporation of straw. Treatment with manure in addition to NPK gave much higher levels of available K than that were achieved with NPK alone. After 1 yr, the level of available K in the M2NPK plot was in excess of 4 mg kg -1 (Fig. 3). Over the course of the experiment, the available soil K increased significantly in soils treated with potassium in the form of either mineral fertilizer or mineral fertilizer plus organic manure, the greatest increase was observed in the M2NPK treatment (14.8 mg kg -1 yr -1 ). By contrast, K levels remained constant in plots where no potassium was added, i.e., those subjected to the CK, N and NP treatments (Fig. 3). DISCUSSION Crop yields The long term yield response on plot receiving NP fertilizers suggested N and P inputs are essential for tested soil to gain high crop yield. The performance of potassium was expected to be indiscernible because the soil investigated is inherently abundance in potassium; the level of exchangeable K in untreated plots (NP) was >16 mg kg -1 after 2 yr intensive cultivation, which was far above the critical value (11 mg kg -1 ) in the studied area (Sun et al. 24). Both annual straw incorporation (SNPK) and supplying 7% of the added N in the form of organic manure (M1NPK) tended to increase the yield of wheat and maize, albeit not significantly, compared to the NP and NPK treatments. These findings are consistent with other researchers who have reported insignificant effects of straw or manure addition based on similar nutrient levels (Nie et al. 212). This may indicate that incorporation of organic amendments for 2 yr under present condition still plays very limited role on crop yield. The addition of 5% more nutrients in the wheat season in the M2NPK treatment did not generate significantly greater wheat or maize yields than those obtained under the M1NPK treatment, which may indicate that the M1NPK treatment provides enough nutrients to make the nutrient supply non-limiting. However, the M2NPK treatment gave greater wheat yield than NP and significantly higher maize yield than both NPK and NP; this was probably due to the input of organic matter, which is known to improve the physical and biochemical properties of the soil (Hati et al. 27; Liu et al. 21; Giacometti et al. 213). The application of NP or NPK afforded high crop yields throughout the experiment and caused significant increases in the wheat yield over time. This may imply that amendment with NP at the rate used in this work is well-suited to the studied cropping system. This is consistent with the general belief that most soils of the Loess Plateau in China are high in K and K is rarely a limiting factor for cereal crops (Su 21). Cai and Qin (26) also reported relatively stable yields with NP and NPK treatments over a 14-yr period on sandy loam soil. However, Jiang et al. (26) reported declines in wheat and maize yields under NP and NPK treatments over a 2-yr experiment conducted at a site with sandy soil; this decline was attributed to an insufficient K supply (available soil K at 51 mg kg -1 ), and was particularly pronounced for wheat. In a 15-yr experiment conducted

7 48 YANG Xue-yun et al. Soil organic matter (g kg -1 ) CK N NK PK NP NPK SNPK M1NPK M2NPK were obtained with treatments involving the combined use of organic amendments and chemical fertilizers (SNPK and MNPK), as found in previous study (Manna et al. 213). Such combinations are likely to be particularly effective because they provide an adequate supply of nutrients and also because the accumulation of organic matter over time improves the physical and biological environment of the soil, favoring crop growth (Barzegar et al. 22; Hati et al. 27; Liu et al. 21; Giacometti et al. 213). Soil Olsen P (mg kg -1 ) Available soil K (mg kg -1 ) Year Fig. 3 Variation in the levels of soil organic matter, soil Olsen P and available soil K in the upper 2 cm of the soil under various fertilization regimes over 2 yr. at a site with red soil (ph 5.7), Zhang et al. (29) observed significant decline in wheat and maize yields under the NP treatment and significant decline in the yield of wheat (but not maize) under NPK. They attributed the reduction under the NPK treatment to soil acidification; those under the NP treatment were attributed to acidification and K deficiency. The most significant increases in wheat yield reported to date Nitrogen use efficiency The lowest RE values of wheat observed in this work were obtained under the N and NK treatments because their meagre grain yields were relative to those treatments received both N and P fertilizers (Tables 1 and 2). The combined application of N and P resulted in a remarkable enhancement in RE (by more than 4%) with respect to the application of N alone or the combination of N and K (Table 2); treatment with K had no discernible effect on RE values during the 2-yr period. In light of the very low yield obtained under the PK treatment, this suggests that both N and P inputs are essential if high yield and high RE value for winter wheat are to be achieved in the tested soil. The significantly lower RE values in the MNPK treatments relative to the other treatments in which NP was employed may reflect a lower availability of the N in organic manure, which provided the bulk of the added N in these treatments, although both M1NPK and M2NPK gave high yield. Similar results have been reported by Yadvinder-Singh et al. (24). The average RE values in treatments involving the application of NP were uniformly above 5%, which is about twice as high as comparable values reported from previous short-term wheat research trials in China (Zhang et al. 28), and very similar to the global average (54%) (Cui et al. 21). This highlights the importance of proper nutrient management in improving RE. The large year-on-year variation in AE values of wheat observed for individual treatments was related to yield fluctuation (Fig. 1), which could have been due to variation in climatic factors (e.g., temperature and precipitation, Fig. 4). The M2NPK and SNPK

8 Trends of Yield and Soil Fertility in a Long-Term Wheat-Maize System 49 treatments yielded significantly lower AE values than M1NPK, NPK and NP, which could be due to the relatively higher quantity of N added in the former treatments, particularly for M2NPK (Table 6). The average AE values (24-28 kg kg -1 ) observed under treatments involving the application of both N and P were much higher than the value of 8 kg kg -1 which was observed in a previous short-term wheat trial in China (Zhang et al. 28). This discrepancy might be due to the lower yield in our control treatment, in which no nutrients were supplied for 2 yr. The average productivity of nitrogen (PFP) value (43 kg kg -1 ) reported by Zhang et al. (28) falls in the upper range of 21 to 61 kg kg -1 given in Table 2. Our results showed that wheat yields increased significantly over the years in plots treated with both N and P (Table 1), as did the PFP values. The average PFP values in some years of our experiment (22, 26, 28-21) were similar to those obtained in short-term experiments and are comparable to the global average PFP for cereals (44 kg kg -1 ) (Dobermann and Cassman 25). However, because the area of land devoted to cereal cultivation is expected to decline in the coming years, it will be necessary to increase the global PFP for cereals at a rate of.1 to.4 kg kg -1 yr -1 in order to meet the expected cereal demand in 225 (Dobermann and Cassman 25). Maize exhibited lower RE values under the NP contained treatments (37-51%) than did wheat (54-66%), despite it has higher grain yields than wheat (Table 1). This could be related to differences in the rate of soil N mineralization, which would affect the availability of N from the soil over the course of the growing season for the two crops. The weather during the maize season tends to be warm and rainy, which favours the decomposition of organic matter and would thus be expected to increase the supply of N to the soil. Such an increase in supply would lead to increased uptake of N by maize in the unfertilized control plot. This inference is consistent with the observation that while the yield of maize is much higher than that of wheat on the control plot, the two crops give much more similar yields under treatments involving the application of N and P (Table 1). Significantly higher RE, AE and PFP values for maize were obtained under treatments incorporating organic amendments, especially M1NPK and M2NPK, compared to those using only inorganic Air temperature ( C) Precipitation (mm) NP or NPK. This may be partially due to the increased supply of N to the soil through decomposition of organic matter in favoured season, and partially to improvements to the physical and biological properties of the soil caused by the long-term incorporation of organic amendments, which would in turn increase maize N uptake (Barzegar et al. 22; Hati et al. 27; Liu et al. 21). Soil fertility Wheat season Maize season Year Fig. 4 Mean air temperature and precipitation for wheat and maize seasons in Yangling, Shaanxi Province, China, from 199 to 29. The SOM content in all treatments significantly increased over time. Increasing SOM levels in unfertilized soils have previously been reported by Yadvinder-Singh et al. (24), and statistically significant increases have been reported by Kaur et al. (28) and Liu et al. (21). The significant increase in SOM levels in the control was probably due to the addition of carbon via the roots and crop residues, an increase in the humification rate constant, and a reduction in the decay rate (Kundu et al. 22). Increasing SOM levels under the N, NK and PK treatments were presumably due to the same factors as mentioned for unfertilized soil above. The heavy accumulation of SOM in the fertilized plots can be attributed to sustained large inputs of organic matter due to increased crop productivity and repeated treatment with straw and manure (Table 1), which were consistent with many other studies in the world (Liu et al. 21; Manna

9 41 YANG Xue-yun et al. et al. 213; Zhou et al. 213). Soil Olsen P contents generally varied between 2 and 3 mg kg -1 after about 6-yr of experiment under NP, NPK and SNPK treatments, which fell in the relative high P fertility level for the tested soil (Fu et al. 21). This indicates that maintaining Olsen P at a relatively high level needs a P application rate of 8 kg ha -1 yr -1, approximately. Applying chemical P fertilizers in conjunction with manure resulted in large Olsen-P concentrations and a heavy positive P balance, which not only resulted in very low P efficiency but also might cause environmental problems by leaching (Yang et al. 24). In addition, phosphate fertilizer is a non-renewable resource. The large Olsen P concentration observed under these combined treatments suggests that it should be advisable to consider the amount of P supplied in manure when deciding how much inorganic fertilizer to apply, in order to utilize phosphate fertilizer and phosphate-containing waste as efficiently as possible. Soil exchangeable K still maintained the original level without K application (Table 5), especially under NP treatment notwithstanding the substantial negative balance of K based on the approach of input-output relationship (Table 4). The similar results were previously presented in northwest and north China by Liu et al. (21) and Tan et al. (212). The seemingly-contradictory phenomenon that soil exchangeable K levels increased on some of the plots receiving potassium fertilizers while the apparent K balance was negative, in accordance with findings by Singh et al. (22) and Bhattacharyya et al. (26). These are presumably due to K uptake by crops, which might draw on K in the deeper soil layers and/or from the non-exchangeable pool. The contribution of K uptake from the subsoil could be considerable (Kautz et al. 213). It has been suggested to account for 41-67% of the total K uptake in green manure crops (Witter and Johansson 21). A lot of evidences have showed that crops use non-exchangeable K (Singh et al. 22; Sharma et al. 21). Decreases in the abundance of non-exchangeable K with simultaneous increases in exchangeable and water-soluble K concentrations suggest that much of the K taken up by crops may come from non-exchangeable species via solution and exchangeable phases in a way that establishes and maintains the equilibrium between the various forms of K in the soil (Singh et al. 22). In addition, there will be some other mechanisms at the bottom of the phenomenon, such as weathering of K bearing soil parent materials (Munson 1985; Tan et al. 212), the release of K into the soil from increased SOM and changed soil ph (Munson 1985). Under present condition, soil ph values dropped from the initial of 8.65 to 8.58 on NPK and 8.2 on M2NPK over 2 yr, and SOM content in all treatments significantly increased over time. The practice of incorporating organic materials (both manure and straw) into the soil is important in maintaining the supply of K, since it mobilizes non-exchangeable K and draws it into the soil solution in addition to serving as an extra source of available soil K. In this work, the wheat and maize yields from the NP plot did not differ significantly from those of the NPK plot over 2 yr. However, the fact that the increase in wheat yields over time is more pronounced in the NPK plot than in the NP plot (Table 1) suggests that replenishment of soil potassium pools is likely become eventually necessary for the long-term maintenance of potassium fertility and consequently high soil productivity even in K-rich soils. CONCLUSION Our results indicate that the application of N and P is crucial for sustaining desirable high yields of wheat and maize for a long run. Application of mineral K currently is not recommended in the tested soil for cereal crops based on a huge buffering capacity of K and economic cost. But the practice of crop residuals incorporation in combination with NP fertilizers is advocated in view of its marked effect on improvement of SOM and beneficial impact on replenishment of soil K and on crop yields. In addition, it should bear in mind that the accumulation of large quantities of P exerts an environmental threat for amendment of organic manure (MNPK) which is conventionally taken as nitrogen rather than phosphorus source. Overall, the results suggest that combinations of appropriate rate of organic manure and mineral nitrogen, or returned straw combined with NP could improve soil fertility and increase the yields achievable

10 Trends of Yield and Soil Fertility in a Long-Term Wheat-Maize System 411 with wheat-maize system and will thus have sound environmental and agronomic consequences on the tested soil. MATERIALS AND METHODS Study site and experimental design A long-term experiment was established in October 199 at the Chinese National Soil Fertility and Fertilizer Efficiency Monitoring Base of Loess Soil ( N, E, altitude m a.s.l.), Yangling, Shaanxi, China. The soil at the site is silt clay loam (clay 32%, silt 52% and sand 16%) derived from loess materials. On average, at the time of establishment the soil at the site contained 7.44 g kg -1 organic C,.93 g kg -1 total N, 9.57 mg kg -1 Olsen-P, 191 mg kg -1 exchangeable K, 92.5 g kg -1 CaCO 3 and had ph of 8.62 across all plots, with low variability (CV 6%, except for Olsen-P, 15%). The experimental site has a mean annual temperature of ca. 15. C, and mean annual precipitation of ca. 55 mm, which mainly falls from June to September (Fig. 4). The field experiment was laid out with a series of 196 m 2 (14 m 14 m) plots, but without treatment replicate for practical reasons. Nevertheless, the entire experimental area was homogenous from the soil physical and soil chemical point of view before the experiment commenced, as previously reported by Yang et al. (212). A winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) double crop rotation was adopted every cropping year for nine treatments of fertilizer/manure applications, which were no added fertilizer or manure (the control treatment, henceforth referred to as CK); various combinations of inorganic nitrogen (N), phosphorus (P) and potassium fertilizers (K), including N, NK, NP, PK, NPK; NPK plus wheat straw or maize stalk (SNPK); and NPK plus dairy manure (two separate treatments, M1NPK and M2NPK; M refers to dairy manure and the following number denotes the rate of manure amendment). Details on the precise quantities of each fertilizer used in the different treatments are provided in Table 6. The SNPK plot received 4 5 kg (air-dried) wheat straw per ha annually between 199 and 1998; since 1999 it has benn instead treated with aboveground parts of maize stalks harvested from the plot in the preceding season, which had a mean annual weight of kg ha -1 (ranging from 2 63 to 5 99 kg ha -1 ). The added straw/stalk was manually chopped into small pieces with lengths of ca. 3 cm and incorporated into the soil in autumn before the sowing of the winter wheat. Similarly, dairy manure was added once per year immediately before the sowing of the wheat. The C and N contents of the manure were about and 1.32%, respectively. The annual mean dry weight of organic manure applied was 13.3 and 2. t ha -1 in the M1NPK and M2NPK treatments, respectively, over the course of the experiment. All inorganic fertilizers and organic materials applied were incorporated into the soil to plowing depth (ca. 2 cm) before the winter wheat was sown and about 1 mon after maize was planted. The N-containing inorganic fertilizer used in the experiment was urea, P was added as single super-phosphate, and K as potassium sulfate. Winter wheat was sown in October and harvested in the following June; summer maize was planted and harvested about 3 mon later, at the end of September or in early October. During the course of the experiment, varieties of wheat and maize changed approximately every 5 yr according to local cultivation, i.e., Xiaoyan 6, Laizhou 953, Shan 253 and Xiaoyan 22 for wheat and Shandan 9, Hudan 4, Gaonong 1 and Zhengdan 958 for maize, respectively. On an annual basis, the plots were irrigated with ground water once or twice during the winter wheat season and to 3 times during the summer maize season, as required; approximately 9 mm of water was applied in each case. All above-ground crop residues were removed after harvesting unless otherwise specified. The fields were conventionally tilled with a rototiller. Sampling and analyses Soil samples were collected within 15 d of the winter wheat harvest each year (in June), from soil depths of -2 cm using an auger with an internal diameter of 2.5 cm. Each Table 6 Details of the different fertilizer treatments and fertilizer application rates (kg ha -1 yr -1 ) in the cropping system Treatment Winter wheat 1) Summer maize N P K N P K CK N NK PK NP NPK SNPK M1NPK M2NPK ) Values after the + represent the mean amount of N/P/K contained in the added crop straw or organic manure.

11 412 YANG Xue-yun et al. sample was a composite of at least 15 soil cores from the same plot. Prior to chemical analysis, the composite soil samples were mixed thoroughly, air-dried, and crushed until they could pass through a 1-mm sieve. SOM levels were calculated by multiplying the measured concentration of soil organic carbon (SOC was determined by the K 2 Cr 2 O 7 - H 2 SO 4 oxidation method) by a factor of Olsen-P was measured by extracting the soil sample with.5 mol L -1 NaHCO 3 (ph 8.5), after which the phosphorus concentration of the extract was determined using the procedure of Li (1983). Soil exchangeable K was extracted using 1 mol L -1 ammonium acetate (ph 7.) and measured using a flame photometer (Li 1983). At crop maturity, crops were harvested manually with sickles cutting close to the ground from three areas with each of 8 and 2 m 2 for wheat and maize, respectively, to estimate their yields for each plot. Subsamples of wheat and maize grain and straw were collected from each plot and dried. Plant samples were ground to pass through a.5-mm sieve and digested with concentrated sulfuric acid/hydrogen peroxide (Li 1983). The total N content of the digest was determined using the Micro-Kjeldahl method, phosphorus was determined by the ammonium molybdate method, K was determined by flame photometry. The samples from precipitation and irrigation water were analyzed for P and K concentrations at the experimental site in by ICP-AES. Calculations Nitrogen use efficiency Long-term N-use efficiency (apparent recovery efficiency (RE); agronomic efficiency (AE); and partial factor productivity (PFP)) were calculated as described by Dobermann and Witt (2). In this work, total N uptake refers to the N uptake by above-ground biomass (grain and straw) only. The detailed equations are as follows: RE (%)=(UN-UN )/FN 1 (1) AE (kg kg -1 )=(GY-GY )/FN (2) PFP (kg kg -1 )=GY/FN (3) Where, UN is the N uptake (kg ha -1 ) in the fertilized plot, UN is the N uptake (kg ha -1 ) in the control plot, and FN is the amount of N applied to the plot including both organic and inorganic forms (kg ha -1 ). GY is the grain yield (kg ha -1 ) in the fertilized plot; GY is the grain yield (kg ha -1 ) in the control plot. Nutrient budgets The total quantity of nutrients removed from the soil and stored in the grain and straw of wheat and maize (i.e., plant N, P and K content) were calculated from the nutrient concentrations and yield data measured every year over the 2-yr study period ( ). Apparent P and K balances were calculated from the inputs and outputs measured in the course of the study. P balance= (P input from mineral fertilizer, manure, straw/stalk and seeds, P from rainfall and irrigation water)-total P removal (P uptake into grain and straw/ stalks) (4) K balance= (K input from mineral fertilizer, manure, straw/stalk, and seeds, as well as K from rainfall and irrigation water)-total K removal (K uptake into grain and straw/stalks) (5) The P and K contents of the manure, straw and irrigation water were measured directly. Rainfall contributions of.127 kg P ha -1 yr -1 and 1.76 kg K ha -1 yr -1 were estimated on the basis of measurements that made at the experimental site in 27-29; the values for each year were adjusted to account for differences in annual precipitation. The P and K contents of the irrigation water were not detectable, and were assumed to be zero in all calculations. The amounts of P and K added to the soil in the form of wheat seed (15 kg ha -1 ) and maize seed (75 kg ha -1 ) were estimated to be.34 and.41% of the mass of wheat grain added for P and K, respectively; the corresponding percentages for maize grain were.26 and.42%, respectively. We assumed that P would not be lost through leaching or otherwise from the soil-plant system. It was assumed that no K-leaching would occur in the soil system if its cationexchange capacity (CEC) exceeded 4 mmol kg -1 (Shen 1998); the measured CEC of the soil at the site was 86 mmol kg -1 (Guo et al. 1992). Statistical analysis Significant differences in crop yields, N-use efficiency and soil fertility parameters between fertilization treatments during experimental period were performed by two-way ANOVA without replication using Microsoft Excel 27 (McDonald 29). The least significant difference (LSD) was used to separate means as follows (Zhang et al. 211): LSD.5 =t.5 2 MSE/n Where, t.5 is the critical t value at.5 probability level, MSE is the mean squares for error, and n is the number of years studied. Simple linear regression analyses of grain yields and soil chemical parameters against years were performed to identify trends (slopes); the P values of the slopes were used to assess the significance of the observed changes in yield or chemical parameters (SPSS ver. 16.). Acknowledgements This study was financially supported by the Special Fund for Agro-Scientific Research in the Public Interest of China (21233) and the 21 Innovation Group Program of Northwest A&F University, China. References Barzegar A R, Yousefi A, Daryashenas A. 22. The effect of addition of different amounts and types of organic materials on soil physical properties and yield of wheat. Plant and Soil, 247, Berzsenyi Z, Gyorffy B, DangQuoc L. 2. 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