Soil phoD-harboring bacteria mediate the responses of phosphorus availability to N addition and mowing among soil aggregates

Phosphorus (P), like nitrogen (N), is a major limiting nutrient for ecosystem structures and functions. Soils in grasslands commonly have limited P availability for organisms, especially under global change (i.e., N deposition) and land-use intensification (i.e., mowing or hay harvest). Soil phoD-harboring bacteria regulate P cycling and maintain P supply in soils. However, it remains unclear how P availability responds to N addition and mowing. The potential microbial mechanisms also require clarification among soil aggregates. We conducted a seven-year field experiment to investigate how N addition at different levels (0, 5, 10, and 20 g N/m−2 y−1) and mowing (unmown and mown) affects soil available P in macro- and micro- aggregates in a temperate grassland in Northeast China. We found that N addition markedly decreased available P in macroaggregates, regardless of mowing. In contrast, available P in microaggregates decreased following N addition under mown but increased at addition levels of 10 and 20 g N/m−2 yr−1 under unmown. Our results also showed that soil available P was positively related to the diversity of phoD-harboring bacteria and phoD gene abundance in macroaggregates, and to alkaline phosphatase activity and phoD gene abundance in microaggregates. These findings suggests that microbial characteristics mediating the response of available P to N addition and mowing vary with soil aggregates. Our study highlights that soil aggregates should be carefully protected if we wish to promote the sustainable development of grassland ecosystems and P supply under a scenario of future global change and land-use intensification.