While the effects of carbon on soil nitrogen (N) cycle have been extensively studied, it is not clearly understood how co-existing macronutrients, such as phosphorus (P), affect the N cycle in agroecosystems. In this study, P amendment effects on nitrification in a fertile agricultural soil were investigated under a typical N-P amendment rate. In a laboratory incubation study, soils were amended with urea, monopotassium phosphate and a mixture of urea and monopotassium phosphate at the same rate. In soils that received no amendments (control), P only, urea only, and urea plus P amendment, nitrification occurred within the first five days, with an average net nitrification rate of 5.30, 5.77, 16.66 and 9.00 mg N kg⁻¹d⁻¹, respectively. Interestingly, nitrification in urea-treated soils was retarded by P addition where a N:P ratio seemed to be a key factor impeding nitrification. This was also supported by the response of ammonia-oxidizing bacteria (AOB), which was more sensitive to P addition than ammonia-oxidizing archaea (AOA). The outcome of this study showed that application of P fertilizer suppressed the nitrification process in urea amended soil, suggesting that a synergistic aspect of N and P nutrient management should be further explored to retard N losses from agricultural systems.

Organic phosphorus (P) is an important potential source of plant P nutrition in agro-ecosystems. It was hypothesized that the soil organic P pools were distinguished one from another by different land-utilization patterns. A total of 38 sites were sampled, to clarify the organic P pool and its distribution in paddy fields, vegetable fields, and orchards. Soil organic P fractions, including labile organic P (LOP), moderately labile organic P (MLOP), moderately resistant organic P (MROP) and highly resistant organic P (HROP) were examined. Results showed that the soil total P (TP) and available P (AP) concentration have enhanced by 138% and 1559%, respectively, over the last four decades. The soil total organic P (TOP) accounted for 21.4% of the TP pool. Soil MLOP dominated the organic P reservoir, irrespective of land-planting pattern. Soil organic P fractions ranked as MLOP > MROP > HROP > LOP. The highest accumulations of TP, AP and TOP were in the vegetable fields, followed by orchards and paddy fields. The vegetable fields had higher LOP and MLOP levels than those of the paddy fields and orchards, whereas the paddy fields exhibited higher concentrations of MROP, and HROP. Soil pH, organic matter and available nitrogen all contributed to the buildup of the organic P pool. It was suggested that soil organic P should be considered preferentially in the management of the plant P nutrient in regional planting systems.

To understand the role of paddy soils in the global carbon cycle, it is necessary to analyze the characteristics of the organic carbon pool at different soil depths. It was hypothesized that soil organic carbon fractions including labile organic carbon fraction I (LOCF-I), labile organic carbon fraction II (LOCF-II), and recalcitrant organic carbon (ROC) distributed differently within the soil profile. In this study, soil was collected from 27 typical rice fields in Guangdong Province, south China. The carbon fractions of the paddy field soils were analyzed and compared over a 0–60 cm depth profile. The relationship between carbon content and the physical and chemical properties of the soils was further analyzed using correlation analysis and structural equation modeling. The results showed that soil total organic carbon concentration in paddy fields was increased by 22.1% during the last four decades. In the soil organic carbon pool of 0–60 cm profile, the proportion of 67.31 to 70.31% in ROC, 21.75 to 22.06% in LOCF-I, and 7.7 to 10.63% was recorded, respectively, indicating that ROC was the dominating fraction. Storage of soil total organic carbon and fractions all decreased with the increase in soil depth. Correlation and path analysis showed that total nitrogen was the main driving factor affecting the soil carbon fractions, whereas pH and soil bulk density indirectly affected the content of carbon fractions by influencing total nitrogen. The results imply the importance of soil total nitrogen in paddy carbon management of rice cultivation.

Phosphorus (P) is an essential element for crop production and a key source of nonpoint pollution in agroecosystems. In this study, we sought to analyze P levels and the factors affecting soil P availability, via P adsorption, in a typical field system that is characterized by the year-round cultivation of vegetables. A total of 190 sites were sampled from vegetable fields in Guangdong Province, South China. Within the research area, average concentrations of 124.49 mg P kg^-1 and 1.55 g P kg^-1 were recorded for available P (AP) and total P (TP), respectively, which are 8.53- and 1.78-fold higher, respectively, than the corresponding values recorded in 1980. The determined P adsorption maximum (Q_m) averaged at 488.38 mg kg^-1, which represents a reduction of 16% compared to the values obtained four decades ago. Accumulations of both TP and AP were found to be negatively correlated with the soil’s maximum adsorption buffering capacity (MBC), although no significant correlations with the soil binding energies (k) and Q_m, were seen. However, soil pH was found to be significantly correlated with k and Q_m. Furthermore, both free Mn oxides (Mn_d) and silt concentrations in the soil were found to contribute to explaining the variations in Q_m. Collectively, the findings of this study provide evidence to indicate that there has been an excessive accumulation of P in the perennial vegetable fields of Guangdong Province over the past four decades, which may have had negative effects on the P supply potential of the soil by reducing the maximum adsorption buffering capacity.