Global patterns of soil available N production by mineralization-immobilization turnover in the tropical forest ecosystems

Available N (N-avail) is important to nurish plant-microbial system and sequestrate carbon (C) in terrestrial ecosystems. For forest ecosystem, N-avail is usually calculated as the sum of N-2 fixation (NN2-fixed), N deposition (N-deposition) and soil available N production (Navail-soil), in which Navail-soil determined the N-avail production and its temporal changes. While, there is still a lack of Navail-soil estimation at the global and regional level due to the temporal and spatial variability of influencing factors, such as climate and soil physicochemical properties. By assembling a dataset of gross rates of soil N mineralization (GR(min)), immobilization of ammonium (NH4+) (GR(ac)) and nitrate (NO3-) (GR(nc)), as well as their corresponding geographic information, climate and main soil physicochemical properties, the Navail-soil produced from organic N (N-org) mineralization and inorganic N (N-inorg) immobilization turnover (MIT) was calculated via building a random forest (RF) model in global tropical forests. The results revealed a good fit between the observed and predicted GR(min) (R-2 = 0.76), GR(ac) (R-2 = 0.77) and GR(nc) (R-2 = 0.67). We further estimated that the total mineralized N, immobilized NH4+ and NO3- was 23.97 (10.48-37.46), 17.98 (5.81-30.15) and 4.86 (1.46-8.26) Pg N year(-1), respectively, leading to the total Navail-soil of 1.13 (-0.95-3.21) Pg N year(-1). Referring to the reported average density of NN2-fixed and N-deposition, the total NN2-fixed and N-deposition was 0.03-0.05 and 0.01 Pg N year(-1), respectively, by producting density and square meter of global tropic forest. Then the total N-avail of global tropic forest ecosystem was 1.18 (-0.91-3.27) Pg N year(-1) (Navail-soil + NN2-fixed + N-deposition). According to the tight stoichiometric relationship between C and N in the production of gross primary productivity (GPP) and soil respiration (R-s), C:N ratio of 31.8-41.9 and 22.7-48.2 was calculated, respectively, which all fall into the C:N ratio range of plants and litter (13.9-75.9) in tropical forest ecosystem. These results confirmed the prediction of Navail-soil production from MIT was in line with theoretic estimates by applying RF machine learning. To our knowledge, this is the first estimation of Navail-soil and the results provide the theoretical basis to evaluate soil C sequestration potential in tropical (e.g. southern America, southeast Asia and Africa) forest ecosystem.