Mineral-associated organic matter (MAOM) is considered a stable reservoir for soil nutrients that influences long-term soil carbon (C) and nitrogen (N) dynamics. However, recent experimental and theoretical evidence shows that root exudates may mobilize MAOM, thereby providing plants and microbes access to a large and N-rich pool. Given the mechanisms underlying MAOM C and N mobilization remain largely untested, we examined direct and indirect pathways by which root exudates destabilize this nutrient pool in laboratory mesocosms. We simulated root exudation with ¹³C-labeled oxalic acid to test whether root exudates are directly capable of mobilizing MAOM from mineral surfaces; and with ¹³C-labeled glucose to test whether indirect stimulation of microbial and extracellular enzyme activity leads to MAOM decomposition. We also tested the potential for oxalic acid and glucose to mobilize MAOM in an additional subset of sterilized soils to clarify the potential for non-microbial pathways of MAOM destabilization.

Over the course of the 12-day MAOM incubation with and without simulated exudates, we measured C cycling (CO₂ respiration rates, ¹³C–CO₂ efflux), N cycling (inorganic N pools, gross N mineralization) and related microbial processes (enzyme activities and microbial community composition via phospholipid fatty acid analysis). Both of the simulated root exudates enhanced MAOM-C mineralization, with cumulative respiration increasing 35–89% relative to the water-only control. Likewise, glucose additions enhanced the production of an exo-cellulase and a chitinase by up to 130% and 39%, respectively, while oxalic acid enhanced oxidative enzyme activities up to 91% greater than control rates. We observed a positive association between glucose-induced shifts in enzyme activities, MAOM-C mineralization, and gross ammonification. Oxalic acid additions were associated with initial increases in fungal relative abundance and in sterile soils appeared to stimulate the release of metals and dissolved organic nitrogen into exchangeable pools. Our results indicate that common root exudates, like glucose and oxalic acid, can significantly increase the turnover and potential release of C and N from MAOM through indirect (e.g., enzyme induction) and direct (e.g., mobilization of metal oxides) mechanisms.

矿物质相关的有机物（MAOM）被认为是土壤养分的稳定储集层，它影响长期的土壤碳（C）和氮（N）动态。但是，最近的实验和理论证据表明，根系分泌物可能动员MAOM，从而使植物和微生物能够进入一个大型且富含N的库。鉴于MAOM C和N调动的基本机制仍未经测试，我们研究了直接和间接的途径，通过这些途径，根系分泌物使实验室中的这些养分池不稳定。我们用¹³ C标记的草酸模拟了根系分泌物，以测试根系分泌物是否直接能够从矿物质表面动员MAOM。和¹³用C标记的葡萄糖测试微生物和细胞外酶活性的间接刺激是否会导致MAOM分解。我们还测试了草酸和葡萄糖在另外一部分无菌土壤中动员MAOM的潜力，以阐明非微生物途径的MAOM不稳定的可能性。

在有和没有模拟渗出液的12天MAOM孵育过程中，我们测量了C循环（CO ₂呼吸速率，¹³ C–CO ₂外排），氮循环（无机氮库，总氮矿化）和相关的微生物过程（通过磷脂脂肪酸分析的酶活性和微生物群落组成）。两种模拟的根系分泌物均增强了MAOM-C的矿化作用，相对于纯水控制而言，累积呼吸增加了35–89％。同样，添加葡萄糖可以分别将外切纤维素酶和几丁质酶的产量提高多达130％和39％，而草酸可以将氧化酶的活性提高至比控制速率高91％。我们观察到葡萄糖诱导的酶活性变化，MAOM-C矿化和总氨化之间呈正相关。草酸的添加与真菌相对丰度的最初增加有关，并且在无菌土壤中似乎刺激了金属和溶解的有机氮向可交换池中的释放。我们的结果表明，常见的根系分泌物，如葡萄糖和草酸，可以通过间接（例如，酶诱导）和直接（例如，金属氧化物的动员）机制显着增加MAOM中C和N的转化和潜在释放。