Nitrogen and phosphorus availability have stronger effects on gross and net nitrogen mineralisation than wheat rhizodeposition

Abstract

Soil nitrogen (N) availability is determined by microbial gross N mineralisation (GNM) and immobilisation, where net N mineralisation (NNM) represents their balance. Plants provide a substantial amount of their photosynthesized C belowground into the soil as rhizodeposition, which can stimulate microbial activity affecting GNM and NNM, but this activity also depends on soil N and phosphorus (P) availability. We examined how N (25 and 100 kg N ha−1 or 44 and 177 mg N pot−1) and P (10 and 40 kg P ha−1, or 18 and 71 mg P pot−1) fertilisation affected microbial N mineralisation in soil planted with two wheat genotypes (Suntop and 249) varying in root biomass and rhizodeposition. We used a continuous 13CO2 labelling method to track plant C rhizodeposition and a 15N pool dilution technique to investigate GNM. We further assessed NNM by comparing N pools in plant and soil at the start and end of the experiment. We observed increased GNM with increased P fertilisation, likely because of P-induced N limitation stimulating microbial mining for N, particularly at the low level of N fertilisation. N fertilisation did not affect GNM but the higher level of N fertilisation reduced NNM, likely because of increased microbial immobilisation of fertiliser N. Our results suggest that GNM was more sensitive to soil N and P availability than to rhizodeposition between wheat genotypes, although at high N fertilisation, rhizodeposition contributed to reduced NNM, likely because rhizodeposition enhanced microbial N immobilisation. We conclude that the relative availability of N and P in soil should be considered for managing GNM and NNM in soil.