In common with other undulating landscapes in Asia, wetland rice (Oryza sativa) on the East India Plateau (EIP) was once confined to hydrologic discharge areas or lowlands, but progressive terracing has now allowed rainfed transplanted rice to encroach upon upland recharge areas, with potential effects on both hydrology and soil fertility. Hydrologic variation down the toposequence and its implications for rice production have been well documented but not the variation in soil fertility. Measurements of soil chemical fertility in seven of 24 EIP districts were used to evaluate variation between and within small watersheds stratified down the toposequence into six land classes that reflect hydrology and land use (three with rice and three without; 36 fields per watershed). We aimed to provide a basis for future research to improve the management of soil fertility. Soils overall were acid, with 14 % of fields requiring liming (pH < 5.0) and 44 % requiring management of further acidification (pH 5.0–5.4). Organic carbon (OC, mean 0.9 %) and cation exchange capacity (CEC; mean 10.7 cmol_c kg^−1) were low. Available phosphorus (P) was mostly very low (mean Bray P 4.3 mg kg^−1) and extractable potassium (K) low to marginal (mean 88 mg kg^−1). Non-rice soils generally had lower pH, OC, and CEC than rice soils but higher P and K. Amongst rice fields, those higher in the toposequence had lower pH, OC, and CEC but more P and K. These results are discussed in the context of nutrient flows in the landscape, leading to the conclusion that terracing uplands has reduced the delivery of sediment-bound P to lowlands where, even with organic P recycling, low inputs of inorganic fertiliser have led soil P to decline and become the primary constraint to the yield of wet season crops. Soil K is on the same trajectory. P and K fertiliser use must increase substantially to sustain the system, which is a challenge for risk-averse subsistence farmers. Field-specific recommendations are needed despite systematic toposequence differences because of variability between fields within land classes.

中文翻译：

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东印度高原之后的土壤肥力及其对生产力，肥料使用和可持续性的影响

与亚洲其他起伏的景观一样，湿地稻（Oryza sativa）曾经被限制在东印度高原（EIP）的水文排放区或低地，但逐渐的梯田现在使雨育的移栽水稻侵占了高地补给区，这对水文和土壤肥力都有潜在影响。向下记录的水文变化及其对水稻生产的影响已得到充分证明，但土壤肥力却没有变化。在24个EIP地区中，有7个地区的土壤化学肥力测量被用于评估从小到大的小流域之间和内部的变化，这些小流域划分为6个土地类别，以反映水文学和土地利用（三个有水稻，三个没有稻田；每个流域有36个田地）。我们旨在为将来的研究提供基础，以改善土壤肥力的管理。土壤整体是酸性的 与14％的字段需要撒石灰（pH≤5.0）和44％的字段需要进一步酸化（pH 5.0 5.4）的管理。有机碳（OC，平均0.9％）和阳离子交换容量（CEC；平均10.7cmol）_c ?? kg ^{?? 1}）较低。可用的磷（P）大部分都非常低（平均Bray P4.3â€？mgâkg ^{??? 1}）和可提取的钾（K）低至边际（平均88â€？mgâ？公斤¹）。非水稻土的pH，OC和CEC通常比水稻土低，但P和K较高。在稻田中，最高位的稻田的pH，OC和CEC较低，但P和K较高。根据景观中养分流动的背景，得出以下结论：梯田高地减少了向低地的沉积物结合磷的输送，即使有机磷再循环，无机肥料的低投入也会导致土壤P下降并成为主要磷。限制了湿季作物的产量。土壤K在相同的轨迹上。磷钾肥的使用必须大大增加才能维持这一体系，这对规避风险的自给农民来说是一个挑战。尽管由于土地类别内田间的可变性，尽管存在系统上的后继差异，但仍需要针对田间的建议。