Influence of aggregate size on phosphorus changes in a soil cultivated intermittently: analysis by 31P nuclear magnetic resonance.

摘要

A pot trial using wet-sieved soil aggregates (>4, 4-2, 2-1, 1-0.5, 0.5-25, and remaining soil <0.25 mm) from a soil that had been cultivated out of permanent pasture and used for winter forage crops for 2 years examined changes in P forms before and after 35 weeks when resown with perennial ryegrass. Soil analyses showed that P was depleted after 35 weeks growth. Changes in P forms were analysed by 31P nuclear magnetic resonance of soil NaOH-EDTA extracts, which removed 98-96% of total P (about 1,080 mg kg-1 in unsieved soil before pasture growth). This indicated that aggregate size influenced the concentrations and forms of P probably via a combination of physical protection and moisture status: orthophosphate, monoesters, diesters and pyrophosphate increased with decreasing size, while phosphonates and polyphosphates were unaffected. The increase in pyrophosphate was attributed to fungal growth, while decreases in orthophosphate and labile organic P (diesters) decreased due to either leaching or mineralization and plant uptake. The largest decrease was associated with orthophosphate, which could be replenished by fertiliser. However, given the soil's high potential for P loss, this should only be done to meet conditions for optimal plant growth as any excess would increase the risk of loss. To further minimize P loss without affecting pasture yield, management should maintain or improve soil structure..