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

Abstract

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.

Fig 1

Effect of long-term fertilization on soil pH (A), organic matter (B), total nitrogen (C), total phosphorus (D), mineral nitrogen (E) and Olsen-P (F) in paddy soils. Error bars represent ± standard deviations; different lower-case letters over the bars indicate significant (P ≤ 0.05) differences according to Tukey's HSD test.

Fig 2

Effect of long-term fertilization on soil C:N (A), C:P (B) and N:P (C) ratios in paddy soils. Error bars represent ± standard deviations; different lower-case letters over the bars indicate significant (P ≤ 0.05) differences according to Tukey's HSD test.

Fig 3

Effect of long-term fertilization on acid phosphomonoesterase (A) and phosphodiesterase (B) activities in paddy soils. Error bars represent ± standard deviations; different lower-case letters over the bars indicate significant (P ≤ 0.05) differences according to Tukey's HSD test.

Fig 4

Effect of long-term fertilization on labile-P (A), moderately labile-P and non-labile-P pools in paddy soils. Error bars represent ± standard deviations; different lower-case letters over the bars indicate significant (P ≤ 0.05) differences according to Tukey's HSD test.

Fig 5. Effect of long-term fertilization on phosphorus uptake in rice crop in paddy soils.

Error bars represent ± standard deviations; different lower-case letters over the bars indicate significant (P ≤ 0.05) differences according to Tukey's HSD test.

Fig 6. Linear regression indicating the relationship between soil C:N:P stoichiometry and phosphorus pools affected by long-term fertilization in paddy soil.

Fig 7. Redundancy analysis of the correlations between the soil C:N:P ratio, phosphorus fractions and phosphatase activities.

Red lines indicate the effect of C:N, C:P and N:P ratios on phosphorus fractions and phosphatase enzyme activities. Blue dashed lines indicate the explanatory variables. Abbreviations: AcP: acid phosphomonoesterase activity; DP: phosphodiesterase activity.

Fig 8

The variance partitioning analysis showing the proportional contribution (%) of soil properties (S), soil C:N:P stoichiometry (C), phosphatase (F) and their interactions on phosphorus fractions.

Fig 9

Structural equation modeling results for the direct and indirect effects of soil C:N:P stoichiometry on P lability and uptake at the Chongqing site (A) and the Suining (B) site. The numbers adjacent to the arrows are standardized path coefficients, which are analogous to relative regression weights, and indicative of the effect size of the relationship. The continuous and dashed arrows indicate direct and indirect relationships, respectively. Gray arrows indicate a non-significant effect. The proportion of explained variance (R²) appears above every response variable in the model. The goodness-of-fit statistics for each model are shown in the lower right corner. Chi/df = 0.86. RMSEA: Root mean square error of approximation.