Plant age and rock phosphate effects on the organic resource quality of herbaceous legume residues and their N and P release dynamics

Abstract

Many tropical agro-ecosystems rely on organic resources for nutrient provision. Various factors affect the nutrient release dynamics of organic resources, including application of external nutrients during their growth and their age or physiological stage at the time of harvest. Understanding relationships between organic resource management factors and the resulting nutrient release dynamics is important for optimizing nutrient recovery by crops and minimizing nutrient losses to the environment. Here, Mucuna pruriens and Lablab purpureus legumes were grown on a ‘slope’ and ‘plateau’ field, treated or not with rock phosphate, and sampled from 12 to 30 weeks after planting. These plant residues were then incubated in a sandy soil under laboratory conditions and mineral N and Olsen-extractable P dynamics were measured for 28 days. Our results showed that plant age had an impact on N, P and polyphenol contents. Application of rock phosphate only altered the P content of the legumes on the ‘plateau’ field, which had a lower soil-available P content. We also observed that mineral N dynamics of the organic resources varied between −62% of the added N released after 28 days, indicating net immobilization, and +23%, indicating net mineralization, with most organic resources showing an initial N immobilization phase. N release was negatively related to the lignin-to-N and (lignin+polyphenol)-to-N ratios of the organic resources. Lastly, we observed that the net release of Olsen P relative to the control soil varied between 0.4 and 4.9 mg kg−1 and that net P release was positively related to the P content of the organic resources. In summary, legume age was the parameter that most strongly influenced the quality of the legume residues, and consequently its N and P release dynamics, with potentially significant consequences for N and P uptake recovery and losses and, ultimately, cropping system sustainability.