We use recent plot-level panel data from Tanzanian smallholder farmers to investigate maize yield responses to inorganic fertilizer under variable soil carbon conditions. Unlike many prior studies which consider total carbon measurements, we focus on active soil carbon, which is a component strongly related to key soil functions, such as nutrient cycling and availability. Active soil carbon is found to be a strong predictor of maize yield response to nitrogen fertilizer. These results highlight important sources of variation in the returns to fertilizer investments across plots and smallholder farmers in the region. When farmgate prices for maize and fertilizer are incorporated into calculations of economic returns, we find that the profitability of fertilizer use is strongly dependent upon farmgate price ratio assumptions: under our most optimistic agronomic response estimates, 39% of farmer plots have an average value-cost ratio greater than 1.5 at a maize-nitrogen price ratio of 0.15. That share drops to 4% at a price ratio of 0.12 and 0% at a price ratio of 0.09. Our findings provide insights into the intertwined biophysical and economic underpinnings of low levels of fertilizer use in Tanzania and elsewhere in the region. Raising active carbon stocks in smallholder systems may be a strategic priority in many areas for incentivizing greater use of inorganic fertilizer, reversing land degradation, and achieving sustainable agricultural intensification.

中文翻译：

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活性土壤碳对肥料使用收益的影响：坦桑尼亚小农户玉米田的经验证据

我们使用坦桑尼亚小农户最近的地块级面板数据来研究在可变土壤碳条件下玉米对无机肥料的产量响应。与许多先前研究总碳量的研究不同，我们专注于活性土壤碳，活性碳是与关键土壤功能（如养分循环和可利用性）密切相关的成分。发现土壤中的活性碳是玉米对氮肥的产量响应的强力预测指标。这些结果凸显了该地区各小块土地和小农户肥料投资回报差异的重要来源。将玉米和化肥的农场门价格纳入经济回报的计算中，我们发现肥料使用的获利能力很大程度上取决于农场门价格比率的假设：根据我们最乐观的农艺响应估计，玉米氮素价格比为0.15时，39％的农民田地的平均价值成本比大于1.5。以0.12的价格比率，该份额下降到4％，以0.09的价格比率下降到0％。我们的发现为坦桑尼亚和该地区其他地区肥料用量低的生物物理和经济因素相互联系提供了见解。在许多领域，提高小农系统中的活性碳储量可能是一项战略重点，以鼓励更多地使用无机肥料，扭转土地退化并实现可持续的农业集约化。价格比率为0.09时为12％和0％。我们的发现为坦桑尼亚和该地区其他地区肥料用量低的生物物理和经济因素相互联系提供了见解。在许多领域，提高小农系统中的活性碳储量可能是一项战略重点，以鼓励更多地使用无机肥料，扭转土地退化并实现可持续的农业集约化。价格比率为0.09时为12％和0％。我们的发现为坦桑尼亚和该地区其他地区肥料用量低的生物物理和经济因素相互联系提供了见解。在许多领域，提高小农系统中的活性碳储量可能是一项战略重点，以鼓励更多地使用无机肥料，扭转土地退化并实现可持续的农业集约化。