Data from: Effects of fertilizer on inorganic soil N in East African maize systems: vertical distributions and temporal dynamics

Fertilizer applications are poised to increase across sub-Saharan Africa (SSA), but the fate of added nitrogen (N) is largely unknown. We measured vertical distributions and temporal variations of soil inorganic N following fertilizer application in two maize (Zea mays L.)-growing regions of contrasting soil type. Fertilizer trials were established on a clayey soil in Yala, Kenya and on a sandy soil in Tumbi, Tanzania, with application rates of 0-200 kg N ha−1 yr−1. Soil profiles were collected (0-400 cm) annually (for 3 years in Yala and 2 years in Tumbi) to examine changes in inorganic N pools. Topsoils (0-15 cm) were collected every 3-6 weeks to determine how precipitation and fertilizer management influenced plant-available soil N. Fertilizer management altered soil inorganic N, and there were large differences between sites that were consistent with differences in soil texture. Initial soil N pools were larger in Yala than Tumbi (240 vs. 79 kg ha−1). Inorganic N pools did not change in Yala (132 kg ha−1), but increased four-fold after two years of cultivation and fertilization in Tumbi (371 kg ha−1). Intra-annual variability in NO3--N concentrations (3-33 μg g−1) Tumbi topsoils strongly suggested that the sandier soils were prone to high leaching losses. Information on soil inorganic N pools and movement through soil profiles can be an important way of assessing vulnerability of different SSA croplands to N losses and determining best fertilizer management practices N application rates increase. Better soil biophysical data will be important when projecting potential environmental effects of a dramatic change to the N cycle in intensifying African croplands.撒哈拉以南非洲（sub-Saharan Africa, SSA）的肥料施用规模呈增长态势，但外源施加的氮素（nitrogen, N）的归宿目前仍尚不明确。本研究在两种土壤类型迥异的玉米（Zea mays L.）种植区，针对肥料施用后的土壤无机氮垂直分布与时间动态展开系统测定。试验分别设置于肯尼亚亚拉（Yala）的黏质土壤与坦桑尼亚通比（Tumbi）的砂质土壤，氮肥施用量梯度为0至200 kg N ha⁻¹ yr⁻¹。研究人员每年采集0-400 cm深度的土壤剖面（肯尼亚亚拉采样时长为3年，坦桑尼亚通比为2年），以分析土壤无机氮库的变化特征；同时每3至6周采集一次0-15 cm的表土样本，用以明确降水与肥料管理措施对土壤植物有效性氮素的影响。研究结果显示，肥料管理措施显著改变了土壤无机氮含量，且两个试验点的差异与土壤质地差异高度吻合。初始土壤氮库规模在亚拉试验点大于通比（分别为240 kg ha⁻¹与79 kg ha⁻¹）。亚拉试验点的无机氮库未发生明显变化，维持在132 kg ha⁻¹；而通比试验点在经过两年耕作与施肥后，无机氮库增长四倍，达371 kg ha⁻¹。通比试验点表土的硝态氮（NO₃⁻-N）浓度年内波动范围为3-33 μg g⁻¹，这一结果强烈表明砂质土壤更易发生高强度的氮素淋失。当氮肥施用量提升时，有关土壤无机氮库及其在土壤剖面中运移的信息，是评估撒哈拉以南非洲不同农田氮素流失风险、确定最优肥料管理措施的重要依据。在预测非洲集约化农田氮循环发生剧烈变化所带来的潜在环境影响时，更完善的土壤生物物理数据将至关重要。