Soil phosphorus dynamic, balance and critical P values in long-term

Soil phosphorus dynamic, balance and critical P values in long-term fertilization experiment in Taihu Lake region Shi Lin-lin, Shen Ming-xing*, Lu Chang-yin, Wang Hai-hou, Zhou Xin-wei, Jin Mei-juan, Wu Tong-dong 1 Suzhou Academy of Agricultural science, Suzhou 215155, P.R.China 2 Institute of Agricultural Science in Taihu Lake District, Suzhou 215155, P.R.China 3 Key Scientific Observation & Experiment Station for Paddy Field Eco-environment, Ministry of Agricultural, Suzhou 215155, P.R.China Abstract P (phosphorus) is an important macronutrient for plant but can also cause potential environmental risk. In this paper, we studied the long-term fertilizer experiment (started 1980 year) to assess the soil P dynamic, balance, critical P value and the crop yield response in Taihu Lake District. To avoid the effect of nitrogen (N) and potassium (K), only the following treatments were chosen for subsequent discussion, including: C0 (control treatment without any fertilizer or organic manure), CNK treatment (mineral N and K only), CNPK (balanced fertilization with mineral N, P and K), MNK (integrated organic manure and mineral N and K); MNPK (organic manure plus balanced fertilization). The results revealed that the response of wheat yield was more sensitive than rice, and no significant differences of crop yield had been detected among MNK, CNPK and MNPK until 2013. Dynamic and balance of soil TP and Olsen-P showed soil TP pool was enlarged significantly over consistent fertilization. However, the diminishing marginal utility of soil Olsen-P was also found, indicating that high-level P application in the present condition could not increase soil Olsen-P contents anymore. Linear-linear and Mitscherlich models were used to estimate the critical value of Olsen-P for crops. The average critical P value for rice and wheat was 3.40 and 4.08 mg kg -1, respectively. The smaller critical P value than in uplands indicated a stronger ability of P supply for crops in this paddy soil. We concluded that no more mineral P should be applied in rice-wheat system in Taihu Lake region if soil Olsen-P is higher than the critical P value. The agricultural technique and management referring to activate the plant-available P pool are also considerable, such as integrated use of low-p organic manure with mineral N and K. Key words: long-term fertilization, soil P dynamic, soil P balance, crop yield, critical P value 1 Introduction Taihu Lake is the third-largest fresh water lake in China and also an ecological fragile area. Phosphorus (P) plays an important role in lake eutrophication (Correll 1998), and the battle against the algae blooms caught the world s attention in 2007 (Guo 2007). P is also a vital factor contributing to crop yields, and the amount of P needed for producing 1 t rice was in the range of 1.8-4.2 kg (Dobermann et al. 1996). Thus, to achieve the maximum ecological and agronomic goals, it is necessary and urgent to clearly understand the dynamic and contents of soil P in Taihu Lake District. With respect to the low recovery and residual effects of applied P, long-term field observation is required to assess crop yield response to P input. Many researchers had realized this problem and focused on the P impact of the long-term fertilization experiment in Taihu Lake region. Qiu et al. (2005) discovered the significant positive increase of TP after 12-year fertilization (mineral P plus pig manure) in rice-rape system of this region, and the apparent P balance (APB) were estimated from -7.2 to 4.3 kg P ha -1 yr -1 (Zhang et al. 2003). The similar TP consistent increase was also found in the another long-term fertilization site (rice-wheat rotation) affiliated to CAS (from 1998), and the APB were from -12.9 to 52.1 kg P ha -1 yr -1 (Lin et al. 2006; Yan et al. 2013b). Moreover, the Olsen-P threshold about P leaching and runoff risk was pointed out, which was 26.0 and 24.8 mg kg -1, respectively (Yan et al. 2013a). In our site, Shan and Wang using P fractionation method to evaluate the P leaching risk of different 1

fertilization treatment and found the integrated mineral and organic manure increased the P leaching risk (Shan et al. 2005; Wang et al. 2006). However, the P dynamic and balance over the long-term differential fertilization were not sufficiently discussed in this site, which were tightly associated with soil P pool, crop yields and nonpoint pollution (Han et al. 2005; Zhang et al. 2014). Critical soil P value for crop yield is generally defined as soil P content above which crop responses should not be expected (Mallarino and Blackmer 1992). This value was also known to vary with soil type, crop category, and climate and so on (Tang et al. 2009). Paddy soil is the major soil type in this region, occupying about 2.3 Mha (Zhang et al. 2009), and rice and wheat are also the conventional crops. As we all know, P transformations were common and diverse in paddy soil, such as adsorption/desorption, dissolution/precipitation, mineralization/immobilization, and so on (Patrick and Mahapatra 1968). These transformations and P application strategies could play important roles in the P uptake and use efficiency of crops under the long-term fertilization condition (Khalid et al. 1979; Shen et al. 2004; Gao et al. 2009). Unfortunately, no published reports involved the critical P value for crops in the framework of the long-term fertilization experiment in Taihu Lake region up to now. Hence, it was interesting and important to understand when P would become the dominant limiting factor if no P was added in a long period, or what would happen if fertilizer P was consistently dosed. The focus of this study is (1) to discuss the crop yield response to differential soil P levels in the long-term field trial in this region; (2) to explore the depletion or residual effect of soil P after application of mineral fertilizer P and organic manure; (3) to determinate the critical value for rice and wheat yield in this paddy soil. 2 Results 2.1 Yield responses to P fertilizer over time Crop yields of CNPK, MNK, and MNPK in this study were found significantly raised over time (Fig. 1-A and B, linear regression, P<0.05), and it could be due to the soil fertility increase (Shen et al. 2007). Yields of C0 and CNK were inclined to decrease but without significance (linear regression, P>0.05), except for wheat yield of C0 (P=0.02). The average yields of rice over 33 years were 5105 (C0), 6405 (CNK), 7065 (CNPK), 7212 (MNK), and 7332 kg ha -1 yr -1 (MNPK). The average yields of wheat were 2164 (C0), 3360 (CNK), 4164 (CNPK), 4319 (MNK), and 4498 kg ha -1 yr -1 (MNPK). absolute crop yields between treatments at different time profiles. We also used ANOVA to analyze the differences of The obvious effects of lack of P on rice and wheat yields emerged in the 16th and 31th years, respectively (Fig. 2, showed by black and red arrows). Considering crop yields over the 33 years, absolute rice and wheat yields of CNK were significantly lower than other treatments, and average absolute differences of crop yields between CNK and CNPK were 660 kg rice ha -1 yr -1 and 803 kg wheat ha -1 yr -1. MNPK until 2013 (Fig. 2, subplot). However, there was still no significant difference among CNPK, MNK and 2.2 Dynamics of soil total P (TP) and Olsen-P A quadratic model was used to fit the soil TP or Olsen-P contents over time (Fig. 3-A and B). It is clear that soil TP and Olsen-P of C0 and CNK permanently decreased over time, and after about 15-20 years the Olsen-P contents were stable around 2 mg kg -1, which was the detection limit of NaHCO 3 -extractable P (Gavlak et al. 2003). Soil TP contents of CNPK and MNPK were increasing over 33 years, but TP contents of MNK began to decline after the 17th year by the predicted model. However, soil Olsen-P contents of CNPK, MNK, and MNPK were all found to decrease after a peak, and the predicted highest Olsen-P contents separately emerged at the 19th, 15th and 23th year. Significant differences of TP/Olsen-P contents among treatments were detected using ANOVA (Fig. 3-A and B, subplot on the right-bottom). The sorted average TP contents were 380.53 (C0), 392.87 (CNK), 636.75 (MNK), 662.46 (CNPK), and 1071.94 mg kg -1 (MNPK), respectively. The average 2

Olsen-P contents amounted to 3.78 (CNK), 3.96 (C0), 17.70 (CNPK), 18.46 (MNK), and 46.60 mg kg -1 (MNPK), respectively. 2.3 Cumulative P balance over time and relationship with soil TP/ Olsen-P The cumulative P balance (CPB) over time showed the two evolution directions of soil P pool, which were the accumulating and exhausting (Fig. 4). For the C0 and CNK, the CPB decreased at a steady rate with the slopes of -28.76 and -33.78 kg P ha -1 yr -1 from 1980 to 2013 (P<0.001), respectively. For the CNPK, the CPB increased continuously with a slope of 54.54 kg P ha -1 yr -1 (P<0.001). Special CPB dynamic occurred in the MNK and MNPK due to shift of organic manure in 1997. From 1980 to 1996, for the MNK and MNPK, the CPB separately rose with a slope of 86.76 and 184.99 kg P ha -1 yr -1 (P<0.001). Whereas the CPB of MNK decreased by 41.33 kg P ha -1 yr -1 from 1997 to 2013, and for the MNPK, the CPB still increased by 69.3 kg P ha -1 yr -1 (P<0.001). Increment of soil total P ( TP) was significantly correlated with CPB except of MNK (Fig. 5-A). The rates of TP over CPB were estimated by the slopes of 0.09, 0.07, 0.09, and 0.15 mg P kg -1, for the C0, CNK, CNPK and MNPK treatments, respectively. CPB were summarized by linear-linear and quadratic models (Fig. 5-B). However, relationships between increment of soil Olsen-P ( Olsen-P) and Soil Olsen-P changes of C0 and CNK were defined as P exhausting process, and the slopes of the linear-linear model (C0: slope1=0.001, slope2=0.017; CNK: slope1=-0.00019, slope2=0.016) indicated that Olsen-P contents were stable after removing 363.10 and 419.20 kg ha -1, for C0 and CNK, respectively (Fig. 5-B). At the same time, Soil Olsen-P changes of CNPK, MNK and MNPK could be defined as P accumulation process, and Olsen-P contents of CNPK and MNPK were regressed by CPB using the quadratic model. relevant to CPB. Nevertheless, the Olsen-P contents of MNK was not significantly 2.4 Critical P value (CPV) for crop yields The estimated CPVs from the two models were not very different, for example, the difference of CPV for rice was 0.49 mg kg -1 and for wheat was 0.55 mg kg -1, respectively (Fig. 6, Table 1). Interestingly, CPV for rice yield was smaller than for wheat yield, no matter what model was used. The averaged CPVs were 3.40 (for rice yield) and 4.08 mg kg -1 (for wheat yield). The difference of CPV between two crops was 1.20 (Mitscherlich) and 0.16 mg kg -1 (linear-linear). Moreover, the Mitscherlich model showed the better coefficient of determination, and the parameter b and c for rice was 4.6 and 0.87 times as that for wheat. 3. Discussion 3.1 Crop yield response It is not surprising that P deficiency would affect crop yield, but it is more important thing to understand when the decrease will happen. In this study, we used P value of ANOVA to estimate this time point, and the significant absolute yield decrease owning to lack of P occurred in the 16th year for wheat and in the 31th year for rice. These results indicated that this paddy soil could provide enough plant-available P for a long time, even without any P fertilizer input. Yields of MNK did not significantly decrease compared to CNPK and MNPK, even after 33 years in this study (Figs. 1 and 2). maintain soil productivity. That s to say integrated with mineral N, K and organic manure could provide enough nutrient to However, long-term oversupply of P would cause environment risks, especially in fragile ecology areas (Zhang et al. 2003; Sharpley and Wang 2014), so applied organic manure should be in line with needs of crops. 3.2 Changes and balance of soil P 3

Dynamics of soil TP could be expected in the long-term fertilization treatment, and the similar increase of TP due to long-term P fertilization was widely found in paddy soils (Lee et al. 2004; Nagumo et al. 2013). The absolute CPB evolution rate over time of the non-fertilization treatment estimated the background ability of P supply in agricultural system, and our results showed stronger in paddy soils. For example, the rate referred to this rice-wheat system in Taihu Lake region was higher than in the wheat-maize system of north China and than in the low-input upland crop system of south-west France (Colombo et al. 2007; Shen et al. 2014). The permanent positive increase of CPB of CNPK and MNPK was due to the excessive P input. changed the rate and triggered P exhausting period for the MNK treatment. Shift of organic manure This phenomenon gave us a hint that low-p organic manure was an alternative without yield loss and with low non-point pollution risk. Furthermore, the different rate of soil TP over CPB was driven by the gradient application of P (Shafqat and Pierzynski 2013), illustrating that higher P input would increase soil TP pool in a shorter period. Olsen-P was generally regarded as plant-available P pool, and we inferred that Olsen-P depletion might be the major reason for wheat yield decline in this study. It was interesting that the relationships of the soil Olsen-P over CPB of the five fertilization treatments were not a straight line, which were widely reported in other studies (Tang et al. 2008; Pei et al. 2010; Shen et al. 2014). The soil Olsen-P exhaustion showed by linear-linear model suggested that crops began to utilize the non-labile P in the P-limited condition. The quadratic curve fitting the relationship between Olsen-P and the CPB of CNPK and MNPK indicated that Olsen-P pool was not enlarged along with extension of TP pool. We hypothesized that there was a diminishing marginal utility of Olsen-P. This founding was in agreement with soil Olsen-P dynamic (Fig. 3-B) and probably mainly due to the available P loss and fixing. Concretely speaking, in terms of oversupply of nutrient through the long-term fertilization (CNPK and MNPK), the P removals by crops could explain partly. Effect of runoff and erosion should be negligible because of the good field facilities, but the leaking and leaching was worth to discuss. Shan et al. (2005) studied the downward transformation of P in this station, and pointed out that after long-term fertilization soil Olsen-P leaching of CNPK and MNPK increased even in the depth of 25-30 cm. On the other hand, more available inorganic P could be fixed into organic or microbial P because of higher N deposition in recent years (Yang et al. 2014), and there was about 27 kg N ha -1 yr -1 wet N deposition in Taihu Lake region (Xie et al. 2008). Moreover, it was also worth considering that soil acidification caused by long-term mineral fertilization transformed labile P to non-labile P bounded with aluminum and iron oxides (Hinsinger 2001; Guo et al. 2010; Zhang et al. 2010). 3.3 Critical P values for crops In this study, we preferred to Mitscherlich model for CPV discussion in consider of the better R 2. The CPV for rice yields was smaller than for wheat, and the bigger parameter b and c of Mitscherlich model indicated the bigger soil plant-available P capacity and ability in rice season. rice-wheat system in paddy soil, the similar result for rice-maize system was considered. In terms of lack of CPV studies for long-term Bai et al. (2013) found CPV was 10.9 mg kg -1 for rice in Chongqing and for maize was 11.1 mg kg -1. As we all know available P could increase in the rice season because of irrigation, the major mechanism could refer to the expansion of plant-available P pool in water-logged condition. To be specific, after a period submergence the precipitated phosphate can be released partly, and the lower reduction also plays an important role in desorption of phosphate from clays, ferric & aluminum oxides and carbonates (Patrick and Mahapatra 1968). Indeed, Bo et al. (2011) sampled soil from this field station and found increasing of Fe-P and Ca 8 -P and decreasing of O c -P and Ca 10 -P after a short-term soil waterlogged-incubation. Notably, the average CPV in our paddy soil was smaller than in other upland soils. For instance, in China, reported average CPVs were 12.0-20.7 and 12.5-21.7 mg P kg -1 for maize and wheat, respectively (Tang et al. 2009; Bai et al. 2013). Colombo et al. (2007) displayed the CPVs for five upland-crops in France ranged from 6.9 to 9.8 mg P kg -1, and the similar results from UK upland were 7-18 mg P kg -1 (Poulton et al. 2013). We 4

hypothesized that paddy soil may represent differential CPV compared to uplands, due to the majority regulation soil factors including ph, soil organic matter, structure, and so on (Johnson et al. 2013; Poulton et al. 2013). Soil P availability is intensely affected by ph, below ph 6.0, P becomes tightly bound with aluminum and iron oxides, and above ph 7.0, P becomes tightly bound with calcium (Hinsinger 2001). In this study, initial soil ph was 6.8 and beneficial to P availability. However, in most papers we used to compare the CPV, the soil ph in upland was above 7.0 and even up to ph 8.6 (Colombo et al. 2007; Tang et al. 2009; Poulton et al. 2013), and the results from Bai et al. were below ph 6 or above ph 7.2 in the four studied sites (Bai et al. 2003). In terms of a competition between decomposition products of OM and P for soil sorption (Guppy et al. 2005), relatively higher SOC contents in this study (14.03 g kg -1 ) were also helpful to soil available P supplying, and the averaged initial SOC contents listed in by Tang et al. was only 6.3-7.1 g kg -1 (other papers for CPV comparison did not showed the SOC). 4 Conclusion In this study, wheat yield was more sensitive than rice, and organic manure can be the substitute of mineral P for both rice and wheat without significant yield loss. but the diminishing marginal utility of Olsen-P was also found in this paddy soil. Long-term fertilization significantly increased soil P pool, The CPV for rice was smaller than for wheat in Taihu Lake region, and the smaller CPVs in paddy than others in uplands showed a stronger ability of P supplying. We suggested no more mineral P fertilizer should be applied if soil Olsen-P content was higher than the CPV in Taihu Lake region, and it was better to activate the plant-available P pool, for example, using some low-p content organic manure instead of mineral P. 5 Materials and methods 5.1 Site descriptions and experimental design The experiments were conducted in the Key Field Scientific Observation & Experiment Station of Suzhou Paddy Soil Eco-environment, Ministry of Agriculture, located in the Taihu Lake District in Jiangsu province, China (31º32 45"N; 120º41 57"E). received 1965 sunshine hours. The annual precipitation of this region is around 1100 mm, and annually The paddy soil of this station is clay loam in texture with hydromica and semectite as the dominating clay minerals, and occupied 70% of total paddy soils in Taihu Lake region (Shi et al. 2004). The long-term experiment on soil fertility started in 1980, and the initial topsoil (0-15 cm) fertility indices were as following: soil organic carbon (SOC) 14.03 g kg -1, total nitrogen (TN) 1.43 g kg -1, total phosphorus (TP) 428 mg kg -1, Olsen-P 8.4 mg kg -1, available-k 127 mg kg -1 (extracted by CH 3 COONH 4 ), and initial soil ph was 6.8. The experiment was a randomized block design, and each treatment replicated in triplicate. In this study, to determine the contribution of soil P to crop yield and to avoid the impact of the N and K shortage on yield, only the following treatments were selected for subsequent analysis, which included: (1) C0, control treatment without any fertilizer and can be used to eliminate effect of soil basic fertility, (2) CNK, only fertilized with mineral N and K, (3) CNPK, balanced fertilization with mineral N, P and K, (4) MNK, integrated with organic manure and mineral N and K, (5) MNPK, organic manure plus balanced fertilization. Area of each trial plot was 20 m 2, but these plots were well split by cement and granite to avoid water and nutrient exchange. destruction. Moreover, we use small soil sampler and return extra soil back to protect topsoil from intense N (urea) application was 90-150 and 120-150 kg N ha -1 yr -1 for rice and wheat depending on years, respectively. Calcium superphosphate and K 2 SO 4 (or KCl, used before 1995) were dosed amount to 55.8 kg P ha -1 yr -1 and 137.5 kg K ha -1 yr -1. 5

Pig manure was used as organic amendment before 1996, and then was substituted by oil rape cake. Average carried nutrient by pig manure was about 91.8 kg N ha -1 yr -1, 68.1 kg P ha -1 yr -1 and 59.1 kg K ha -1 yr -1, and by oil rape cake was about 57.6 kg N ha -1 yr -1, 13.5 kg P ha -1 yr -1 and 14.9 kg K ha -1 yr -1. Rice (Oryza sativa) and wheat (Triticum aestivum) were typical field crop in Taihu Lake region and were chosen in all most of years. However, two crops of rice were grown in 1981, and naked barley (Hordeum vulgare var. nudum) was grown rather than wheat in 1982 and 1984. Moreover, in 1994 and 1995, oil rape (Brassica chinensis), and in 2000, broad bean (Vicia faba) were substitutes of wheat. For this reason, we lost wheat yields in these years and use the average wheat yield of two adjacent years instead. 5.2 Soil sampling and analysis Soil samples were collected from 0-15 cm layer after rice or wheat harvesting. In each experiment plot, at least 5 soil cores were sampled using a stainless-steel soil sampler with 2.8 cm inner-diameters. Soil samples were transferred to laboratory and air-dried as soon as possible. Olsen-P was extracted with 0.5 mol L -1 NaHCO 3 (soil was sieved into 2.0 mm), and TP was digested by H 2 SO 4 -HClO 4 (soil was sieved into 1 mm). then determined by the molybdate-ascorbic acid method for determination (Murphy and Riley 1962). Phosphate was 5.3 Statistical analysis In order to reduce yield variability caused by weather conditions (Colwell 1963), the absolute yield was obtained by subtracting the yield of control plots from the yield of the fertilized plots. Then the relative increased yield was used to evaluate the critical P value (CPV) for crop yield and defined as following equations: Y abs =Y-Y 0 (1) Y rel =Y abs /(Y max -Y 0 ) 100 (2) Where, Y abs is the absolute crop yields (kg ha -1 ), Y rel is the relative increased yield (%), Y is the yield of fertilizer plots (CNK, CNPK, MNK, and MNPK), Y 0 is the yield of C 0, and Y max is the maximum observed yield among treatments (Tang et al. 2009). Numerous models had been proposed for determining appropriate the CPV for crop yield (Nelson and Anderson 1977), and to enable comparison with other published data, in this study, we use linear-linear, and Mitscherlich (exponential) models to assess it. The linear-linear model was defined by equations (3) and (4) as Y = a 1 +b 1 X if X<C (3) Y = a 2 +b 2 X if X C (4) Where, Y is the predicted relative increased yield (%), X is the soil Olsen-P (mg kg -1 ), and CPV could be defined as C. The Mitscherlich model was defined by equation (5) as Y=A[1-e -c(x+b) ] (5) Where, Y is the predicted relative increased yield (%), X is the soil Olsen-P (mg kg -1 ), A is the asymptotic relative yield (%), and b and c are soil P supplying capacity and rate parameters which were estimated by maximum likelihood (Poulton et al. 2013). relative increased yield (Colomb et al. 2007). We determined CPV for crop yield associated with 90% maximum predicted Annual apparent P balance (APB) was calculated by subtracting the output P from input P applications, and output P included straw and grain P. We used the ratio of straw to grain to estimate straw quantity which was calculated based on sunshine dried biomass (average water content from 6 to 15%), which ratio was 0.9 for rice and 1.1 for wheat (Bi et al. 2009). The cumulative P balance (CPB) was defined as the sum of APB over years. All discussion about soil P balance in this study were based on 0-15 cm plow layer. The increment of soil P contents ( TP or Olsen-P) was defined by equation (6) as TP/ Olsen-P = P i P 0 (6) 6

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Fig. 2 ANOVA of absolute crop yields showed in the different time-profiles, and the ANOVA began from year 1984 for enough replicates (n>3). The red and black arrows indicated the time point from which significant differences of absolute rice and wheat yield would be found at 0.05 levels, respectively. The subplot showed the significant differences and multiple comparisons of absolute crop yields integrated of 33 years using LSD (0.05). Fig. 3 Dynamics of soil TP/Olsen-P over time fitted by quadratic curve in Taihu-Lake region represented in the figure (A) and (B), respectively. The multiple comparison between treatments showed with boxplots in each figure, and the different upper-case letters represented significant differences at the level of 0.001. Fig. 4 Evolution of the cumulative P balances (CPB) over years (1980-2013) of the five different long-term fertilization treatments in Taihu Lake region. 10

Fig. 5 The effect of cumulative phosphorus balance (CPB) on changes of soil TP (A) and soil Olsen-P (B). In plot (A), a linear model was used to estimate the efficiency of TP balance. In plot (B), for C0 and CNK, a linear-linear model was proper to fit the Olsen-P exhausting, and for CNPK and MNPK, a quadratic model was more useful to illustrate the changes of Olsen-P in the accumulating process. However, for MNK, there is no significant relationship between CPB and changes of TP or Olsen-P (P>0.05). Fig. 6 The response of relative increased crop yield to soil Olsen-P, (A) for rice, (B) for wheat. Table 1 Models for estimate the threshold value of soil Olsen-P for rice and wheat relative yield Crop Model Equation R 2 Degree of freedom Threshold value (mg kg -1 ) Rice Wheat Mitscherlich y=95.44(1-e (-0.41(x+2.49) ) 0.72 57 3.15 linear-linear y=8.14x+63.51 y=0.08x+92.77 0.52 56 3.64 Mitscherlich y=94.08(1-e (-0.47(x+0.54) ) 0.90 53 4.35 linear-linear y=15.40x+31.85 y=0.13x+89.80 0.78 52 3.80 11