Long-term-fertilization effects on soil organic carbon, physical properties, and wheat yield of a loess soil

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1 J. Plant Nutr. Soil Sci. 2011, 174, DOI: /jpln Long-term-fertilization effects on soil organic carbon, physical properties, and wheat yield of a loess soil Xueyun Yang 2, Pingru Li 2, Shulan Zhang 1,2 *, Benhua Sun 2, and Chen Xinping 1 1 State Key Laboratory of Soil Erosion and Dryland Farming, Northwest A & F University, Yangling, , Shaanxi, China 2 College of Resources and Environment, Northwest A & F University, Yangling, , Shaanxi, China Abstract The large dryland area of the Loess Plateau (China) is subject of developing strategies for a sustainable crop production, e.g., by modifications of nutrient management affecting soil quality and crop productivity. A 19 y long-term experiment was employed to evaluate the effects of fertilization regimes on soil organic C (SOC) dynamics, soil physical properties, and wheat yield. The SOC content in the top 20 cm soil layer remained unchanged over time under the unfertilized plot (CK), whereas it significantly increased under both inorganic N, P, and K fertilizers (NPK) and combined manure (M) with NPK (MNPK) treatments. After 18 y, the SOC in the MNPK and NPK treatments remained significantly higher than in the control in the top 20 cm and top 10 cm soil layers, respectively. The MNPK-treated soil retained significant more water than CK at tension ranges from 0 to 0.25 kpa and from 8 to 33 kpa for the 0 5 cm layer. The MNPK-treated soil also retained markedly more water than the NPK-treated and CK soils at tensions from 0 to 0.75 kpa and more water than CK from 100 to 300 kpa for the cm layer. There were no significant differences of saturated hydraulic conductivity between three treatments both at 0 5 and cm depths. In contrast, the unsaturated hydraulic conductivity in the MNPK plot was lower than in the CK plot at depths of 0 5 cm and cm. On average, wheat yields were similar under MNPK and NPK treatments and significantly higher than under the CK treatment. Thus, considering soil-quality conservation and sustainable crop productivity, reasonably combined application of NPK and organic manure is a better nutrient-management option in this rainfed wheat fallow cropping system. Key words: saturated hydraulic conductivity / soil bulk density / soil water retention / total porosity / unsaturated hydraulic conductivity Accepted November 8, Introduction The utilization of organic agricultural materials, particularly organic manure, has been a fairly common practice (Shen et al., 1997), as it not only enhances soil organic matter (SOM) level directly, but also indirectly improves the physical and biological properties of soil (Hati et al., 2007) by reducing its bulk density, increasing its water-holding capacity, improving the soil structure and enhancing microbial activity. The application of inorganic fertilizers is a widely practiced method for increasing crop yields. Their effect on the soilphysical environment is to increase both the aboveground and root biomass through the immediate supply of plant nutrients in sufficient quantities, which in many cases leads to an increase in SOM content (Haynes and Naidu, 1998; Hati et al., 2007). However, on the Loess Plateau, Zhang et al. (2006a) found no difference in the SOM content between inorganically fertilized (NPK) and unfertilized control treatments in experiments conducted under dryland conditions in a silt-loam soil in the middle part of the Loess Plateau. In a sandy-loam soil, Sheeba and Kumarasamy (2001) observed no effect of inorganic fertilizers on soil physical properties, and Schjonning et al. (2005) reported no effect of inorganic fertilization on soil hydraulic conductivity in the plow layers of a sandy loam. These results suggest that the effectiveness of inorganic fertilizers on soil physical/hydraulic properties may depend on the soil type under study and other abiotic factors, e.g., climatic conditions. China has a large area of dryland in the NW of the country accounting for 56% of the nation s total land area, of which a large part ( 40%) is situated on the Loess Plateau (Li, 2004). The dominant cropping system in the region, covering 56% of the arable land, is one of winter wheat followed by a summer fallow (Zhu, 1989). Information on the physical/ hydraulic properties of soils under dryland wheat fallow systems is relatively rare, especially under the temperate climate of the Loess Plateau. In order to facilitate future strategies for formulating sustainable dryland production, it is essential to understand the effects that mineral fertilization and organic manures may have on the organic-c dynamics and hydraulic properties of the soils, as well as any effects on crop yields. We therefore conducted a long-term experiment to obtain valuable information on the effects of various fertilizer treatments on a dryland wheat fallow system over a period of 19 y. * Correspondence: Dr. S. Zhang; address: zhangshulan@nwsuaf.edu.cn

2 776 Yang, Li, Zhang, Sun, Xinping J. Plant Nutr. Soil Sci. 2011, 174, Materials and methods 2.1 Study site and experimental design A long-term experiment on winter wheat (Triticum aestivum L.) followed by soybean or summer fallow was commenced in the autumn of 1990 at the southern edge of the Loess Plateau ( N, E, 534 m asl). The soil at the site is a silt loam (Eumorthic Anthrosols) derived from loess materials. The site has a mean annual temperature of 13.0 C and a mean annual precipitation of 550 mm. During the experimental period ( ), precipitation in winter wheat seasons and summer seasons had no significant change (p > 0.05), but mean temperatures, mean minimum temperatures, and mean maximum temperatures in wheat seasons showed significant increasing trends (p = , p < 0.001, and p < 0.01 for the three temperatures, respectively) (Fig. 1). Before the fertilizer treatments were carried out, three successive crops of winter wheat and summer maize had been grown with no fertilizers or manure being applied over the whole area in order to render the soil fertility as uniform as possible. The final maize crop was terminated at the end of August before maturity. The field experiment was planned with the large plot area (21 m long, 19 m wide) but without treatment replicates due to practical reasons although this design could lead to statistical problems. However, a uniform fertilizer management and cropping system for hundred years, three seasons of homogenized planting as well as the small variability (C.V. < 4%) of the representative parameters of soil fertility in the treatment plots (Tab. 1) were indicating to a low spatial heterogeneity of the field. We therefore believe it reasonable to assume any significant differences later observed between plots to have been caused by the different treatments. The present study included the following three treatments: (1) a control treatment (CK) without any fertilizer inputs; (2) applications of mineral N, P, and K fertilizers (NPK); and (3) applications of dairy manure (M) plus NPK fertilizers (MNPK). In treatment 2, the annual application rates of N, P, and K were 135.0, 47.1, and 56.0 kg ha 1, respectively. In treatment 3, the total input of N was also kg ha 1, but the percentage contribution of N was 70% from the dairy manure and 30% from mineral fertilizer. The quantities of mineral P and K fertilizers were applied as those in treatment 2. Dairy manure was incorporated into the soil annually in the autumn prior to the sowing of winter wheat. The average N content in the Figure 1: Precipitation in winter wheat seasons and summer periods and air temperatures of wheat seasons in Yangling, Shaanxi Province, China from 1990 to Table 1: General soil properties before the experiment started in 1990 (0 20 cm soil depth). Treatments SOC /gkg 1 Total N /gkg 1 Olsen-P /mgkg 1 Available K a /mgkg 1 Total porosity /% CK NPK MNPK Mean C.V. b / % a Available K was extracted with 1 mol NH 4 AC; b C.V. is coefficient of variation Bulk density /gcm 3

3 J. Plant Nutr. Soil Sci. 2011, 174, Soil properties and wheat yield following long-term fertilization 777 manure was (1.71 ± 0.87)% (SD) and the mean dry weight of applied manure was (7.37 ± 4.95) t (SD) during the experimental years. All chemical fertilizers were applied before the winter wheat was sown, and N, P, and K were applied as urea, single superphosphate and K sulfate, respectively. From 1991 to 1998, the cropping system was winter wheat followed by soybean, with two crops per year; however, due to crop failure caused by the frequent occurrence of droughts at the time of soybean planting and seedling from June to early July (absolute failure of soybean in the years of 1991 and 1996 and poor-yield responses in 3 out of 8 y), since 1999 a cropping pattern of one crop per year was implemented with winter wheat followed by a summer fallow. Thus, winter wheat sown in October was harvested in the following June and was then either followed by soybean without applying any fertilizers or by a fallow season lasting about 3 months over the summer. All aboveground crop residues were removed after harvest, and the plots were conventionally tilled. 2.2 Soil sampling and analyses Composite soil samples to a depth of 20 cm were taken from all treatment plots each year after the wheat harvest. The results presented in this paper refer to samples taken in 1990 (start year), 1992, 1995, 1997, 1999, 2001, 2004, 2006, and 2008 and analyzed for soil organic C (SOC) content. In June 2008, 18 soil cores (2.5 cm diameter auger) from each treatment were collected randomly at depth of 0 60 cm (layered as 0 10, 10 20, 20 40, and cm), six of the cores were bulked to form 3 pseudoreplicates representing each treatment. The soil samples were air-dried and passed through a 1 mm sieve to measure the SOC content (Walkley and Black, 1934) and wilting points in a pressure chamber. At 2 m from the plot border, 10 intact soil cores sampled with a stainless-steel cylinder (7 cm, 5 cm in height) were also taken randomly from each treatment at depths of 0 5 and cm. Half of these were used to measure the saturated hydraulic conductivity and unsaturated hydraulic conductivity (using the same soil cores); the other half were used to determine water-retention curves, bulk density, and total porosity. Soil water retention was measured as the volumetric water content after equilibration to each of a range of pressure heads (0, 0.25, 0.75, 2, 5, 8, 20, 30, 50, 100, 300, and 1500 kpa) by standard laboratory techniques. Then, measurements of h(w) were fitted to the model of van Genuchten (1980) S e ˆ 1 1 awš n m ; 1 where S e ˆ h h r h s h r, indexes s and r indicate saturated and residual values of the volumetric moisture content (h), a is a parameter, m =1 1/n, and w is tension (kpa). h s was taken from measured data, and h r, a and n were obtained through fitting by the least-square technique by the solver in Excel. Saturated hydraulic conductivity was determined by the constant-head method (Klute and Dirksen, 1986), and unsaturated hydraulic conductivity was determined by the hot-air method (Arya et al., 1975) combined with the slope of the water-retention curve. The hot-air method was performed using an 1100 W hair-drier (20 min) blowing perpendicularly 4 5 cm above the surface of each tested soil core, after it had been equilibrated with a pressure of 100 kpa. By cutting the core into thin slices and determining the moisture content of each slice, a moisture-distribution curve h(z) can be established, from which the diffusivity [D(h] can be calculated as a function of moisture content as: D h ˆ 1 dz 2t dh h i h zdh; where t is the total drying time, z is the distance from the evaporating surface, and h i is the initial water content (by volume). Diffusivity was calculated following the procedure of Gieske and De Vries (1990). The unsaturated hydraulic conductivity was subsequently obtained by multiplying D(h) by dh/dw the slope of the soil water-retention curve at the point being considered, i.e., K h ˆD h dh dw : Total porosity was determined in undisturbed 200 cm 3 watersaturated samples, assuming no air was trapped in the pores. The soil moisture contents in the field were recorded gravimetrically at 105 C for 24 h, using five replicates cored at 0 10 cm, cm, cm, cm, and cm, and sampled once every second week from March 22, 2004 until June 24, 2004 during the winter wheat growing season. 2.3 Data analysis All statistical analyses were conducted using SPSS (SPSS Inc, Chicago, Illinois, USA). Means and standard deviations or standard errors are reported for each of the measurements. ANOVA was used to assess the effects of the fertilization treatments on the measured variables. When ANOVA indicated a significant F value, multiple comparisons of means were performed using the least-significant-difference method (LSD). 3 Results and discussion 3.1 Soil organic carbon Soil organic carbon dynamics Changes in SOC as a function of time were significantly affected by fertilization treatments, although there were large fluctuations (Fig. 2). The SOC content of the control soil remained unchanged during the 18 y of the experiment, as indicated by a linear regression slope not significantly different from zero (B = , R = 0.224, p = 0.563). Overall, the application of NPK fertilizers significantly increased SOC content with time: the linear regression slope was g kg 1 y 1 (R = 0.770, p = 0.015). Manure plus NPK fertilizers 2 3

4 778 Yang, Li, Zhang, Sun, Xinping J. Plant Nutr. Soil Sci. 2011, 174, Figure 2: Dynamics of soil organic C contents in treatments of CK (no fertilizer), NPK (chemical fertilizer), and MNKP (chemical fertilizer + manure) in the long-term experiment from 1990 to (MNPK) further enhanced the SOC content and gave a linear regression slope of g kg 1 y 1 (R = 0.867, p = 0.002). The control soil maintained its initial SOC level despite having no fertilizer input for 18 y. Similar results were reported by Fan et al. (2008) and Prakash et al. (2007). This was probably due to the relatively high levels (> 20 kg N ha 1 ) of deposition of either dry or wet materials from the atmosphere that occurs in this region (Wang, et al., 2008). This could have provided crops with some macronutrients such as N, so enhancing to some extent the input of organic materials into the system through increased crop stubbles and root biomass, and compensating for any losses due to the decomposition of SOC. The plot receiving only inorganic fertilizers (NPK) had a significantly greater SOC, which might be explained by the higher amount of root residues and stubbles of all the crops being added to the soil from the considerably higher crop yields in that plot (Ghosh et al., 2003; Hati et al., 2006). In contrast to the CK and NPK treatments, the significant increase in the SOC content of the MNPK plot was attributed to the direct addition of organic matter from manure and the greater amount of root residues and stubbles of all the crops also being added to the soil (Hati et al., 2007). The temporal trend of changing SOC in plots receiving NPK and MNPK were similar to the results reported by Cai and Qin (2006) of similar studies conducted in the Huang-Hui-Hai Plain of China. These results indicated that long-term chemical NPK fertilization alone or MNPK combination can significantly increase soil C sequestration, especially the latter one. and MNPK plots tended to be somewhat higher than in the CK plot. In the present study, the addition of organic manure to NPK substantially influenced the SOC content in the plow layer (0 20 cm) whereas compared with the control, inorganic fertilizers alone markedly affected the SOC only in the upper 10 cm soil layer. By contrast, Rasool et al. (2008) reported that 32 y of NPK fertilization of a sandy-loam soil under tropical climate condition improved the SOC to a depth of 45 cm in an irrigated maize wheat cropping system. This difference might be explained by the use of a more intensive cropping system and possibly SOC leaching on a sandy-loam soil in the latter study. Other studies have also shown that continuous-cropping systems sequester more SOC than fallow systems (Sherrod et al., 2003; Machado et al., 2006). 3.2 Soil bulk density and total porosity In general, soil bulk density was lower in the 0 5 cm than in the cm soil horizon. The bulk density of the MNPK Soil organic carbon in soil profile After 18 y of treatment with fertilizer and dairy manure, the SOC content of the MNPK-treated soil was significantly higher (p 0.01) than that of the control and NPK-treated soils both in the 0 10 cm and the cm horizons (Fig. 3). At a depth of 0 10 cm, the NPK plot also showed a marked enhancement of SOC compared to the CK plot, but no difference was observed between these two plots at a depth of cm. At the deeper soil horizons of cm and cm, no significant differences were found between any of the three treatments, although the SOC levels in the NPK Figure 3: Soil organic C contents at depths 0 10 cm, cm, cm, and cm of soils subject to CK, NPK, and MNKP treatments after 17 y. Vertical bars indicate ± 1 standard deviation.

5 J. Plant Nutr. Soil Sci. 2011, 174, Soil properties and wheat yield following long-term fertilization 779 Table 2: Saturated hydraulic conductivity (cm d 1 ), soil bulk density (g cm 3 ), and total porosity (%) at the different soil depths. Treatment Soil depth K sat Bulk density Porosity /cm /cm d 1 /gcm 3 /% CK (81.2) a 1.14 (0.10) 59.8 (3.3) NPK (55.0) 1.13 (0.10) 60.5 (4.6) MNPK (54.5) 1.08 (0.07) 63.2 (2.5) CK (49.1) 1.53 (0.04) 48.3 (0.8) NPK (29.2) 1.51 (0.11) 49.7 (3.9) MNPK (43.5) 1.34 (0.06) 55.2 (4.1) a Numbers in parentheses are standard deviations. treated soil was significantly lower than either the NPK-treated or the CK soils at a depth of cm. In the 0 5 cm horizon, the soil bulk density in the MNPK plot was slightly lower than in the other two plots, but there were no significant differences between plots at this depth (Tab. 2). In contrast, the soil bulk densities of the NPK and CK plots were similar in both horizons. The responses of the soil bulk density are related to increased pore volume and decreased particle density in soils amended with organic manure (Schjonning et al., 1994). The trends of total soil porosity between treatments were contrary to those of bulk density (Tab. 2). The MNPK-treated soil with a lower bulk density had significantly higher porosity values in both horizons than the soils from the CK and NPK plots (p 0.1 at 0 5 cm and p 0.01 at cm). The NPKtreated soil had porosity values similar to those of the CK soil. The higher total porosity in the MNPK soil may be due to its higher SOM content (Fig. 3), change in the distribution of pore sizes (Fig. 4), and better aggregation in the soil (Aggelides and Londra, 2000). This is also consistent with Celik et al. (2004) who found that soil total porosity increased with the application of fertilizer and compost. 3.3 Soil water retention The MNPK-treated soil retained significantly more water than the CK soil in the 0 5 cm horizon at tensions from 0 to 0.25 kpa (p < 0.05) and from 8 to 33 kpa (p < 0.05), and considerably more than the NPK-treated and control soils in the cm horizon at tensions from 0 to 0.75 kpa (p < 0.05) and from 100 to 300 kpa (p < 0.1) (Fig. 4). The permanentwilting point was not affected by the annual application of manure at either soil depth; nor were there any substantial differences of soil water retention between the NPK and CK treatments (p > 0.1). Fitted parameters for the van Genuchten model (van Genuchten, 1980) are presented in Tab. 3. The comparable n values between treatments at both depths suggested that the slopes of the water-retention curves were similar. The parameter n depends on soil texture, which is related to soil particle-size distribution, and is almost unaffected by management practices (Ndiaye et al., 2007). The a values for the MNPK-treated soil were significantly higher than for the NPK-treated and control soils at a depth of 0 5 cm, and were much higher than for the control soil at a depth of cm. Other authors have obtained similar results (Ndiaye et al., 2007). Evidently, management practices do exert a pronounced impact on the air-entry pressure (a parameter). Water retention is primarily controlled by the number of pores, their size distribution, and the specific surface area of the soil (Hillel, 2004). After being manured annually for 17 y, the water retention of the manure-treated soil was higher than the CK soil by 2% to 13% across the whole tension range ( kpa) at a soil depth of 0 5 cm. Manuring increased the numbers of pores in all pore-size classes, significantly enhancing the number of pores < 37.5 lm in size (equivalent pore diameter), and indistinctively increasing the number of pores > 37.5 lm. In the cm horizon, manure application substantially increased the number of pores >150 lm and < 3 lm, but reduced the number of pores between 3 lm and 150 lm in size. These results are in agreement with those of Schjonning et al. (1994) who found that organic man- Figure 4: Soil water retention from soil subject to CK, NPK, and MNPK treatments at depths of 0 5 cm (top panel) and cm (bottom panel). Bars indicate ± 1 standard error (n = 5).

6 780 Yang, Li, Zhang, Sun, Xinping J. Plant Nutr. Soil Sci. 2011, 174, Table 3: Parameters from the van Genuchten model fitting (m =1 1/n) for the different soil depths. Treatment Soil depth /cm h s a /cm 3 cm 3 h r /cm 3 cm 3 a n / kpa 1 CK (0.03) (0.057) (0.065) (0.099) NPK (0.05) (0.074) (0.022) (0.132) MNPK (0.03) (0.053) (0.093) (0.053) CK (0.01) (0.041) (0.054) (0.083) NPK (0.04) (0.075) (0.222) (0.072) MNPK (0.04) (0.019) (0.244) (0.017) a h s is the saturated water content, h r is the residual water content, a and n are shape factors of the van Genuchten model. Numbers in parentheses are standard deviations. ure significantly affected the volume of pores < 30 lm ata depth of 0 20 cm on a sandy-loam soil. However, Miller et al. (2002) reported long-term manure application to have no significant effect on soil water retention in the 0 5 cm horizon, but to significantly increase water retention at most water potentials between 0 and 1500 kpa in the cm soil horizon under dryland farming on a clay-loam soil. Schjonning et al. (1994) and Miller et al. (2002) also found the volume of pores < 0.2 lm to differ significantly between treatments with and without manure application, but in our case a similar increase was not significant. When compared with CK, the NPK treatment in contrast to MNPK did not significantly influence water retention at any soil depth that we examined. This is according to the findings by Schjonning et al. (1994) and Mbagwu (1992). The effect of NPK treatment on water retention could be related to its limited role in raising SOC content (Fig. 1). 3.4 Hydraulic conductivity Measured saturated hydraulic conductivity, K sat, varied greatly, yielding large standard deviations (Tab. 2), and so did not differ significantly between the treatments in either of the tested soil horizons. Similar results have been reported elsewhere (Shirani et al., 2002; Wu et al., 2003). When measuring saturated conductivity, Miller et al. (2002) also found that treatments with added manure gave variable results on dry land with significant differences in one year and no difference in another. Since saturated water flow in the soil is dominated by the macropore flow (Lipiec and Hatano, 2003), the similar K sat values observed between treatments in the present study might be explained by similarities in the macropore structure of the different plots. The volume of pores > 37.5 lm in the 0 5 cm horizon did not differ significantly between treatments; neither did the volume of pores > 400 lm at a depth of cm. Unsaturated hydraulic conductivity, however, clearly differed between the MNPK and the CK treatments at both soil depths tested (Fig. 5) where the water content was > 0.20 cm 3 cm 3. No significant difference in unsaturated hydraulic conductivity was found between the CK and NPK treatments at either depth. Unsaturated hydraulic conductivity was smaller in the MNPK-treated soil than in the control when the soil was relatively wet (> 0.2 cm 3 cm 3 ), although the MNPK treatment had a higher amount of pores < 3 lm in both depths (Fig. 4). Figure 5: Unsaturated hydraulic conductivities of soil subject to CK, NPK, and MNPK treatments at depths of 0 5 cm (left panel) and cm (right panel).

7 J. Plant Nutr. Soil Sci. 2011, 174, Soil properties and wheat yield following long-term fertilization 781 Blair et al. (2006) also observed lower unsaturated conductivity in a manure-managed plot at low tension in the Rothamsted Broadbalk experiment. Zhang et al. (2006b) reported similar trends under irrigated conditions at the same experimental site that was used for the present study. Possible explanations for these findings may be that the addition of manure might have caused the soil to develop hydrophobic properties; or that the necks of some soil pores might have become clogged by organic matter, so decreasing the connectivity between pores. Because the addition of manure decreases unsaturated conductivity, it could help to reduce water loss and hence be of benefit to wheat growth under dry-land conditions. 3.5 Seasonal variations in soil water content Dynamics of soil moisture during the winter wheat growing season is shown exemplarily for the year 2004 in Fig. 6. In the 0 10 cm horizon, significantly higher water content in MNPK than in CK was observed on all sampling dates with the exception of May 18, and markedly higher water content in NPK than in CK on three sampling dates was also detected (March 22, April 1, and April 29). On average, soil water content was 28% and 18% higher in MNPK and in NPK than in CK, respectively, over the season. In the cm horizon, water content in MNPK was only significantly higher than in other treatments on one sampling date (April 1). In the other soil layers (20 30 cm and cm), the water content was similar in all treatments. Miller et al. (2002) measured the effect of added cattle manure on seasonal changes in soil water content and found that, compared with a control, manuring significantly increased the soil water content at a soil depth of 0 15 cm. In the present study, we found fairly good agreement between seasonal soil water content, soil water retention, and unsaturated hydraulic conductivity, at all the soil depths investigated. The similarities found in the water content at the deeper soil layers (20 30 cm and cm) of all treatments are expected since the SOC contents of the lower horizons are affected by fertilizer management to a far lesser degree than of the upper horizons (Fig. 3). Hence, the SOC has less influence on the soil s physical properties and consequently the water content at these depths. These in situ measurements of soil water content throughout the season give a more accurate indication of the dynamic soil water status and potential water availability to the crop than the static concepts of soil water retention. Thus, the higher crop yields of the fertilized treatments compared to the control plot corresponded with higher water contents in the upper soil depth. This effect was more pronounced with manure amendment which had also increased the soil water retention. 3.6 Wheat grain yield Wheat yields were analyzed in two phases, i.e., in the wheat soybean phase ( ) and in the wheat fallow phase ( ) (Tab. 4). Yields were significantly higher in the NPK and MNPK treatments than in the CK treatment, no matter in which phase of the experiment yields were taken (Fig. 7). In the wheat soybean phase, yields in MNPK were either lower or similar compared to the yields in NPK, on average, there was no significant difference between the two treatments. In the wheat fallow phase, yields were higher in MNPK than in NPK in 6 out of 10 years, on average, MNPK produced the same yield as NPK did. The coefficients of variation (CV) among years (Tab. 4) showed that the wheat soybean phase was noticeably less stable than the wheat fallow phase. In addition, the yields of Figure 6: Seasonal water content of soil subject to CK, NPK, and MNPK treatments at depths of (a) 0 10 cm, (b) cm, (c) cm, and (d) cm. Vertical bars indicate ± 1 standard deviation (n = 5).

8 782 Yang, Li, Zhang, Sun, Xinping J. Plant Nutr. Soil Sci. 2011, 174, Figure 7: Wheat grain yield in treatments of CK, NPK, and MNPK in wheat soybean phase ( ) and wheat fallow phase ( ) in the long-term experiment in Yangling, Shaanxi Province, China. the NPK and MNPK treatments were oscillating in the wheat fallow phase with an increasing trend which is obvious in Fig. 7 although it was not significant (Tab. 4). Hence, present climate condition supported the wheat fallow cropping system better than the wheat soybean cropping system. One reason can be seen in the organic proportion of N of 70% that was applied only in the MNPK plot in the form of the organic manure with a C : N ratio > 20. As a result of the induced competition from microbial decomposers for available N, the availability of mineral N for crop growth might be lower in the MNPK plot compared to the NPK treatment. Hence, wheat yields were depressed on MNPK treatment compared with the NPK treatment in the wheat soybean phase. Later, with the built up of SOC (Fig. 2) and N pools, and with improved soil-physical environment, such as enhancement of soil water retention (Fig. 4 and Fig. 6), the yield gap between NPK and MNPK diminished. Nevertheless, the CV value was still higher in MNPK (20.2) than in NPK (16.9) in wheat fallow phase, and this might be partly explained by the variation of SOC contents in the manure, which influence the biological mobilization/immobilization of N and therefore the N availability; and partly interpreted by the reason that yield in MNPK was more dependent on weather variables than in NPK. Correlation analysis showed that wheat yield was significantly correlated (p 0.1) with growing-season precipitation and annual precipitation (rainfalls in previous fallow plus precipitation in wheat season), and was related, but not significantly, with the mean minimum temperature of wheat season (p = ). Such correlations were not detected in NPK treatment. These correlations in MNPK implied that low intensities of nutrient supplies upon organic mineralization impact on wheat yield when precipitation and minimum temperature were low. This might suggest that the appropriate ratio is rather important in management strategy for combining application of organic and inorganic fertilizers. As a N source applied like in this study, the organic N should account for a small portion of N-application rate in the beginning period and then gradually increase its portion over time, which could prevent decreasing crop productivity from low intensity of N supply by organic manure, and meanwhile guarantee the benefit effect of soil-structure modification by manure addition and stabilize higher crop productivity. In the long run, taking into consideration the water limitation normal in dry-land areas, an adequate combination of organic manure and inorganic fertilizer might be one of the most desirable management practices due to improvements of SOC sequestration and soil-physical environment, although NPK alone could also maintain relatively high wheat yields. 4 Conclusions Table 4: Means and linear trends of wheat yields as affected by long-term fertilization. Analysis of the results of a 19 y trial has shown that chemical- NPK application to some extent contributed to C sequestra- Treatment mean yield /kg ha 1 C.V. / % yield trend /kg ha 1 y 1 R p mean yield /kg ha 1 C.V. / % yield trend /kg ha 1 y 1 R p CK 1303 b a b NPK 3390 a a MNPK 2928 a a a Different lowercase letters within the same column indicate significant differences between treatments at p < p means significant level of yield trends.

9 J. Plant Nutr. Soil Sci. 2011, 174, Soil properties and wheat yield following long-term fertilization 783 tion, while the combination with manure in the MNPK treatment resulted in a substantial build-up of SOC compared with CK and NPK treatments. In addition, the MNPK treatment reduced soil bulk density, increased total porosity, improved soil water retention, and decreased unsaturated hydraulic conductivity compared to both CK and NPK treatments or to the control alone. Overall, wheat yields were similar between MNPK and NPK treatments and both were significantly higher than in the control treatment. Considering soil-quality conservation and sustaining crop productivity, a combined application of inorganic fertilizer and organic manure is a better nutrient-management option in the long run in this rain-fed wheat fallow cropping system than inorganic fertilization alone. 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