Distribution and determinants of organic carbon and available nutrients in tropical paddy soils revealed by high–resolution sampling

Rice is one of the most important crops in China; however, the loss of paddy area and imbalanced soil nutrient composition threaten food security in China owing to the increasing food demand. Soil nutrients significantly influence soil fertility, quality, and productivity, thereby controlling rice production and agroecosystem sustainability. However, a systematic assessment of the nutrient distribution and its driving factors is lacking in the low–yield paddy soils of southern China. In this study, 3239 surface paddy soil samples covering an area of 1278 km2 were collected and analyzed for soil organic carbon (SOC), alkaline hydrolyzable nitrogen (AN), available phosphorus (AP), and available potassium (AK). The dominant factors influencing SOC, AN, AP, and AK were identified using a Random Forest (RF) method. We identified deficient levels of SOC (8.94 g kg−1), SOC density (SOCD, 21.1 t ha−1), AN (55.3 mg kg−1), and AK (37.2 mg kg−1) in tropical paddy soils, but high levels of AP (34.7 mg kg−1). According to the RF, SOCD was significantly correlated with soil AN and AK, implying their coupled cycling in tropical paddy soils; however, the negative correlation between SOC and AP signified decoupling. Compared with waterloggogenic paddy soils, gleyed paddy soils had significantly higher SOC concentrations and densities but significantly lower AP and AK concentrations and pH. Analyses of cropping system effects indicated that double– and triple–cropping systems had significantly higher SOC, bulk density (BD), SOCD, AP, AK, and pH than those of single–cropping systems. These results suggested that tropical paddy soils can potentially sequester OC and available nutrients via efficient soil management. Considering the deficient AN and AK levels of single–cropping system and their high importance on SOCD and soil productivity, double– and triple–cropping systems (generally with high N and K fertilization and residue return) are recommended as an efficient approach to improve the poor soil nutrient status and enhance the OC pool and rice production in tropical paddy soils.