Altered soil nitrogen retention by grassland acidification from near-neutral to acidic conditions: A plant-soil-microbe perspective

Grassland ecosystems are facing soil acidification, occurring either rapidly or gradually, alongside the persistent threats of acid deposition and nitrogen (N) pollution. However, the understanding of how grassland soils retain N in response to acidification remains limited, despite its critical importance in providing essential services in these ecosystems. Considering the prevalence of near-neutral or weak-acidic (e.g., pH 6) soils in grasslands worldwide, we aim to predict the pathways through which grassland acidification alters soil N retention. Firstly, as soil fungal proportions increase, the carbon:nitrogen ratios of the microbial community also rise, leading to a downregulation of microbial N uptake and retention. Secondly, grassland acidification weakens the stability of soil organic matter (SOM) by physically and chemically dissolving its components, but also conserves SOM by hindering biological mineralization, thereby enhancing N retention. Thirdly, acidification damages 2:1 clay minerals and causes occupation of exchangeable positions by H⁺, liberating fixed ammonium and subsequently attenuating grassland soil N retention. Fourthly, acidification leads to the breakup of macroaggregates, exposing SOM to microbial attack but also compromising microbial habitats and activity, resulting in uncertainty in grassland N retention. Fifthly, dominant plant stress-tolerators allocate more N aboveground as less decomposable defensive metabolites, reducing the risk of N loss during grassland litter degradation. Overall, in future research, it is crucial to focus on anticipating the changes in soil N retention under acidification, highlighting the synergistic or antagonistic interactions among the reviewed grassland modulators/processes, and quantifying the impacts of declined N deposition on grassland soil N retention and cycling, given the current and recent past stable or even declined atmospheric N deposition in many regions.