Soil organic matter (SOM ) is a key indicator of soil fertility, and building SOM is assumed to decrease reliance on external inputs and ensure stable crop production. Recent syntheses of field data support this assumption with positive SOM –productivity relationships that asymptote at ~4% SOM . Teasing out the directionality of this relationship—the extent to which SOM increases crop growth vs. greater growth leading to higher SOM concentrations—requires controlled experimentation. To disentangle this causative pathway, we conducted a greenhouse experiment whereby we manipulated SOM concentrations from 1% to 9% and evaluated whether the SOM –productivity relationship differed for spring wheat (Triticum aestivum, L.) under nitrogen fertilization crossed with irrigation due to the expectation that SOM buffers the effects of reduced fertilization and/or irrigation. We found that higher concentrations of SOM led to greater productivity (measured as aboveground biomass) up to a threshold of 5% SOM , after which productivity declined across all treatments. These declines occurred despite the fact that indicators of soil health (water‐holding capacity, microbial biomass, and bulk density) improved linearly with increasing SOM concentrations. That is, improvements in soil properties did not translate to gains in productivity at the highest SOM levels. Nitrogen fertilization led to greater productivity across all treatments, but to a greater relative extent at lower SOM levels, where we found that productivity on unfertilized soils with 4% SOM matched that of fertilized soils with 2% SOM . Differences in productivity on unfertilized soils due to irrigation emerged at higher SOM levels (>5%), highlighting SOM 's role in water retention. Our results demonstrate that building SOM leads to improved growth of a globally important crop; however, our results also indicated a pronounced SOM threshold, after which crop growth declined. This underscores the need to develop optimal SOM targets for desired agricultural and environmental outcomes.

中文翻译：

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使用受控温室研究的直接证据表明土壤有机质对作物生长的阈值影响。

土壤有机质（SOM）是土壤肥力的关键指标，并且假设建立SOM可以减少对外部投入的依赖并确保稳定的作物生产。最近的现场数据合成以正的SOM与生产率的关系支持了这一假设，SOM与生产率的关系渐近于约4％的SOM。弄清这种关系的方向性（SOM促进作物生长的程度与更高的生长导致更高的SOM浓度的程度）需要受控的实验。为了弄清这种致病途径，我们进行了温室试验，将SOM的浓度从1％调整为9％，并评估了春小麦（Triticum aestivum，L.）由于期望SOM缓冲减少的施肥和/或灌溉的影响，因此在氮肥下与灌溉交叉。我们发现较高的SOM浓度导致更高的生产率（以地上生物量衡量），最高达到5％SOM阈值，此后所有处理的生产率均下降。尽管土壤健康指标（持水量，微生物生物量和容重）随SOM浓度的增加而线性改善，但这些下降还是发生了。也就是说，土壤特性的改善并没有转化为最高SOM水平下的生产率提高。施氮可提高所有处理的生产率，但在较低的SOM水平下可提高相对生产率，在这里我们发现SOM为4％的未施肥土壤的生产力与SOM为2％的施肥土壤的生产力相匹配。在较高的SOM水平（> 5％）下，由于灌溉而导致的未施肥土壤的生产力差异出现了，这突显了SOM在保水方面的作用。我们的结果表明建立SOM可以改善具有全球重要性的农作物的生长。然而，我们的结果也表明明显的SOM阈值，此后作物生长下降。这强调了需要针对所需的农业和环境成果制定最佳的SOM目标。但是，我们的结果也表明明显的SOM阈值，此后作物生长下降。这强调了需要针对所需的农业和环境成果制定最佳的SOM目标。然而，我们的结果也表明明显的SOM阈值，此后作物生长下降。这强调了需要针对所需的农业和环境成果制定最佳的SOM目标。