Research Article: Soil carbon (C), nitrogen (N) and phosphorus (P) stoichiometry drives phosphorus lability in paddy soil under long-term fertilization: A fractionation and path analysis study

Date Published: June 24, 2019

Publisher: Public Library of Science

Author(s): Muhammad Qaswar, Waqas Ahmed, Huang Jing, Fan Hongzhu, Shi Xiaojun, Jiang Xianjun, Liu Kailou, Xu Yongmei, He Zhongqun, Waleed Asghar, Asad Shah, Huimin Zhang, Paulo H. Pagliari.


Soil C:N:P stoichiometry plays a vital role in nutrient cycling in ecosystems, but its importance to P transformation in paddy soil remains unclear. We investigated the effect of soil C:N:P stoichiometry on P mobility and uptake under long-term fertilization. Three treatments, CK (no fertilization), NPK (inorganic nitrogen, phosphorus and potassium fertilization) and NPKM (combined inorganic NPK fertilizer and manure application), were selected from two long-term experiments of paddy soil that were initiated in 1991 and 1982 in Chongqing and Suining, respectively. The results showed that in comparison the control treatment, under long-term fertilization, soil pH decreased. In comparison with the NPK and CK treatments, the NPKM treatment significantly increased soil nutrient contents, P uptake and phosphatase activities. In comparison to the CK treatment, the NPK and NPKM treatments significantly decreased soil C:N, C:P and N:P ratios. In comparison to NPK and CK treatments, the NPKM treatment decreased residual-P at both sites. Compared with CK treatment, the NPKM treatments increased labile-P and moderately labile-P by 987% and 144%, respectively, and NPK treatment increased these factors by 823% and 125%, respectively, at the Chongqing site. At the Suining site, with NPKM treatment, increases in labile-P and moderately labile-P were 706% and 73%, respectively, and with NPK treatment, the increases were 529% and 47%, respectively. In contrast, non-labile-P was significantly decreased with NPKM treatment in comparison to that with NPK and CK treatments. Moreover, increases in soil C:N and C:P ratios decreased the labile-P pools and increased non-labile-P pools. A path analysis indicated that soil C:N:P stoichiometry indirectly controlled P uptake by directly affecting P transformation from non-labile to labile-P pools. Moreover, the non-labile-P in soil with high SOM and P content directly affected P uptake, indicating that soil P transformation is mainly driven by soil C and P in paddy soil. In conclusion, understanding mechanism of P mobility influenced by soil C:N:P stoichiometry could be helpful to manage soil P fertility under long-term fertilization in paddy soils of these regions.

Partial Text

Crop productivity is directly affected by P availability in agricultural soils [1]. However, in some agricultural soils, the concentration of total P is naturally high, which can limit plant growth due to the low solubility and rapid conversion of P compounds to unavailable or poorly available P after fertilizer application [2]. A synthetic chemical fertilizer application can ensure sufficient availability of P for plant uptake [3]. However, there has been growing confirmation that long-term excessive application of inorganic P fertilizers decreases P use efficiency that may result in excessive accumulation of P in soil [4].Specifically, in intensive cultivation areas, during surface runoff or erosion the P-rich soil particles may reach surface water and cause eutrophication [5,6].

We concluded that in comparison with no fertilization and chemical fertilization, long-term combined application of manure and chemical fertilizers significantly increased soil total and available nutrient content. In comparison to the CK and NPK treatments, the NPKM treatment significantly increased phosphatase activities in both soils. The soil C:N and C:P ratios was highest in the CK treatment, and no significant difference was observed between the NPK and NPKM treatments for the C:N ratio at both sites. While, the lowest soil C:P and N:P ratios were observed in the NPKM treatment at both sites. The NPKM treatment significantly increased P mobility and uptake, and reduced residual-P at both sites via increasing SOM. SEM pathways provided new insights into P lability mediated by the soil C:N, C:P and N:P ratios. The C:N, C:P and N:P ratios had a direct effect on moderately labile-P and non-labile-P, thus indirectly affecting mobile-P and P uptake. The soil organic matter and soil P content at the Chongqing site were relatively higher than those at the Suining site. Therefore, non-labile-P at the Chongqing site directly affected P uptake, and at the Suining site indirectly affected P uptake, indicating that soil C and P were two most important driving factors of P lability in paddy soil. Our findings suggest that soil nutrient stoichiometry is very important for managing the soil P fertility in paddy soil under long-term fertilization in these regions.




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