Phosphorus availability and soil biological activity in an Andosol under compost application and winter cover cropping

In Andosols, available P for crops is limited primarily by sorption and precipitation processes, but application of organic materials may improve the P availability by enhancing organic P mineralization. A field study was conducted during 2005–2007 on a Silandic Andosol in Fukushima, Japan, to investigate whether and how applications of composted cattle manure (0, 61, and 122 or 183 kg P ha−1) and/or cover-crop residue (no cover crop, rapeseed, and cereal rye) would improve P availability to soybean. Cover crops were grown over winter and incorporated into the soil 2 weeks before compost application. Soybean P uptake at flowering was improved by application of compost and/or rye residue. Bray-2 soil P (i.e., readily soluble and desorbable P) increased only in the compost treatment. Soil phosphatase activity and microbial P, both representing the potential of P mineralization, were enhanced in treatments with compost and rye. Rapeseed had minor effects on the soil P parameters. Soil nematode community structure was evaluated as an indicator for soil conditions including decomposition pathways. Compost application increased free-living nematodes, especially fungal-feeding nematodes, implying that fungal decomposition dominated in the soil. The increase in free-living nematodes was less pronounced in the cover-crop treatments than in the compost treatment; only bacterial-feeding nematodes consistently increased after the rye treatment. The different changes in the community composition of soil nematodes and the P parameters indicate that the use of a rye cover crop affected P availability to soybean differently to compost application. The density of Pratylenchidae, the prevailing plant feeder in the soil investigated, declined in the treatments with compost and rye, but increased in the rapeseed treatment. The feeding activity of Pratylenchidae may also have affected root growth and consequently P uptake by soybean.