Sustainable cropland management requires preservation of soil organic matter (SOM). In spite of in depth understanding gained from ample field and laboratory studies, we have a poor understanding of landscape scale spatial variation of fresh organic matter (OM) decomposition and its conversion into soil organic carbon (SOC). Particularly, local topographic position may be expected to co-control these processes via soil hydrology. In this study, we sought to identify if such control is significant by setting up a field experiment with two contrasting positions across 10 gently sloping cropland fields covering three different soil texture groups, i.e. loamy sand, (sandy) loam and silt loam. We wanted to link OM decomposition to within-field differences in soil moisture, whilst keeping variation in other soil and management factors minimal. Specifically, mesocosms with C enriched ryegrass (the OM source) were incorporated in the fields for ten weeks and afterwards, soil was separated into > 500 µm, 53 – 500 µm and < 53 µm sized fractions. Overall, we found that lower located positions were wetter than higher positions with average differences of 11 %, 20 % and 16 % in water-filled pore space for the loamy sand, (sandy) loam and silt loam soil, respectively. Mineralization of added OM was surprisingly independent of landscape position, even though moisture conditions appeared wetter than optimal at the low but not at the high landscape positions. Remaining ryegrass residues > 500 µm did follow local topography-driven gradients in soil moisture with higher amounts in low landscape positions. In other words, drier conditions at high landscape positions improved coarse OM decomposition, with consequently more ryegrass-carbon (C) ending up in finer soil fractions (< 500 µm). Additionally, soil texture affected decomposition of the smallest fraction (< 53 µm) with a stabilizing effect for finer-textured (silt loam) soils. We conclude that, despite significant contrasts in moisture conditions between landscape positions, within-field spatial variability of OM mineralization was overall limited during the observed wet summer period. Nevertheless, landscape position affected the quality of remnant unmineralized C, with relatively more conversion of freshly added OM into OM associated with silt and clay at the drier higher positions, potentially improving the long-term stability of SOM. Likewise observations under different weather conditions are needed to evaluate the necessity of precise modelling of local soil hydrology for predicting SOC stock evolution on the landscape scale.

中文翻译：

---

潮湿夏季土壤湿度控制景观位置对有机质分解的影响


可持续农田管理需要保护土壤有机质（SOM）。尽管通过大量的现场和实验室研究获得了深入的了解，但我们对新鲜有机物（OM）分解及其转化为土壤有机碳（SOC）的景观尺度空间变化知之甚少。特别是，当地的地形位置有望通过土壤水文共同控制这些过程。在这项研究中，我们试图通过在覆盖三个不同土壤质地组（即壤土、（沙）壤土和粉壤土）的 10 个缓坡农田中设置两个对比位置的田间试验来确定这种控制是否显着。我们希望将 OM 分解与田间土壤湿度差异联系起来，同时将其他土壤和管理因素的变化保持在最低限度。具体来说，将富含 C 的黑麦草（OM 来源）的中生态系统纳入田间十周，然后将土壤分为 > 500 µm、53 – 500 µm 和 < 53 µm 大小的部分。总体而言，我们发现，壤质砂土、（砂质）壤土和粉质壤土的充水孔隙空间的平均差异分别为较低位置比较较高位置湿润，分别为 11%、20% 和 16%。令人惊讶的是，添加的 OM 的矿化与景观位置无关，尽管在低景观位置的湿度条件似乎比最佳状态湿润，但在高景观位置则不然。剩余的黑麦草残留物 > 500 µm 确实遵循当地地形驱动的土壤湿度梯度，在低景观位置含量较高。 换句话说，高景观位置的干燥条件改善了粗有机质分解，因此更多的黑麦草碳 (C) 最终形成更细的土壤部分 (< 500 µm)。此外，土壤质地影响最小部分（< 53 µm）的分解，对细质地（粉质壤土）土壤具有稳定作用。我们的结论是，尽管景观位置之间的湿度条件存在显着差异，但在观察到的潮湿夏季期间，OM 矿化的田内空间变异总体上是有限的。然而，景观位置影响了残余未矿化碳的质量，在干燥的较高位置，新添加的有机质转化为与淤泥和粘土相关的有机质的转化相对较多，有可能提高SOM的长期稳定性。同样，需要在不同天气条件下进行观测，以评估对当地土壤水文进行精确建模以预测景观尺度上 SOC 库演变的必要性。