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

Abstract

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/yr. Soil profiles were collected (0–400 cm) annually (for three years in Yala and two 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). Inorganic N pools did not change in Yala (277 kg/ha), but increased fourfold after cultivation and fertilization in Tumbi (371 kg/ha). Intra‐annual variability in NO−3‐N concentrations (3–33 μg/g) in 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 h vulnerability of SSA croplands to N losses and determine best fertilizer management practices as N application rates increase. A better understanding of the vertical and temporal patterns of soil N pools improves our ability to predict the potential environmental effects of a dramatic increase in fertilizer application rates that will accompany the intensification of African croplands.

Deep soils modify environmental consequences of increased nitrogen fertilizer use in intensifying Amazon agriculture

“…Hydrologic separation of water used by plants from water that rapidly passes through soils to groundwater is likely widespread 48 – 50 . This has also been observed on Kenyan Oxisols, where lysimeters fail to sample most soil solution that drains rapidly through the soil macropores 51 , 52 .…”

“…This suggests that most of the increase in extractable nitrate in deep soils occurred during previous 10 years of soybean cropping, with no detectable increase thus far under the double-cropping system, which had been practiced on that field for only two years. Storage of extractable nitrate in deep soils has also been observed in central Amazon forests 53 and agricultural Oxisols of Kenya 51 , suggesting that these soils have anion exchange capacity that allows them to hold large quantities of nitrate, in contrast to less weathered temperate soils in which nitrate is typically highly mobile 54 – 56 . High nitrate pools at several meters depth after logging, in perennial crop culture 53 , 57 , and after the addition of added 15 N-labeled fertilizer 58 have been observed in similar soils in the adjacent Cerrado region of Brazil.…”

Nitrogen rate impacts on tropical maize nitrogen use efficiency and soil nitrogen depletion in eastern and southern Africa

“…While soil N mineralization probably also occurred in Kiboko and Harare, these two sites also had a greater tendency towards N being recalcitrant as there was less total organic N in the rooting profile. Embu soil’s low moisture and pH characteristics may have also limited how much additional N was lost to leaching (Tully et al 2016 ).…”

“…In addition, crop residues are also used as feed and, therefore, are often not available for soil fertility improvement. It is crucial, therefore, to determine how applying moderate (< 100 kg N ha −1 ) and low (< 60 kg N ha −1 ) amounts of N impacts grain yield, especially in a continuous maize cropping system, which is more dependent on higher N input levels than maize in rotation and is commonly utilized in eastern and southern Africa (Tully et al 2016 ).…”

Toward a nitrogen footprint calculator for Tanzania

“…Loss of soil fertility also drives the clearing of forests to gain new fertile land for crop production, thus aggravating the erosion-pollution problem (Leip et al 2014a). N fertilizer applications are expected to increase in sub-Saharan Africa and the sandy soils of Tanzania may predispose farm lands to fertilizer loss, although soil profiles vary widely (Tully et al 2016). Advocating for efficient use of N fertilizers and reducing N loss through the food production system becomes even more important as the use of fertilizer increases.…”