Abstract
Flooded paddy soil ecosystems in the tropics support the cultivation of the majority of the world’s leading crop, rice, and nitrogen (N) availability in the paddy-soil rooting zone limits rice production more than any other nutritional factor. Yet, little is known about the dynamic response of paddy soil to N-fertiliser application, in terms of horizontal and vertical patchiness in N distribution and transformation. Here, we present a microscale analysis of the profile of ammonium (NH4+) and nitrate (NO3−), nitrification, oxygen (O2water and O2soil), and pH (pHwater and pHsoil) in paddy soils, collected from two representative rice-production areas in subtropical China. NH4+ and NO3− exhibited dramatic spatiotemporal profiles within N patches on the microscale. We show that pHsoil became constant at 1.0–3.5 mm depth, and O2soil became undetectable at 1.7–4.0 mm. Fertiliser application significantly increased pH, and decreased O2, within N patches. Path analysis showed that the factors governing nitrification scaled in the order: pHwater > pHsoil > NH4+ > O2water > NO3− > O2soil. We discuss the soil properties that decide the degree of nutrient patchiness within them and argue that such knowledge is critical to intelligent appraisals of nutrient-use efficiencies in the field.
Highlights
Soils, which are further accentuated by spatiotemporal fluctuations
Nitrification, which is performed by ammonia-oxidising bacteria (AOB)[19] or ammonia-oxidising archaea (AOA)[20,21,22,23] converting NH4+ to nitrite (NO2−), and by nitrite-oxidising bacteria (NOB)[24] converting the latter to nitrate (NO3−), is a key process in the global N cycle
N concentrations within fertiliser patches can be expected to be manifold higher than in the bulk soil. It remains unanswered what scale and magnitude such differences might assume in waterlogged paddy soils and how the spatial variations of NH4+, NO3−, nitrification activity, pH, O2 and their temporal changes contribute to the spatial variation of the total inorganic N pool
Summary
Soils, which are further accentuated by spatiotemporal fluctuations. The microzone around such intensively applied N fertiliser displays significant variations in ion concentrations, pH, chemical N transformation rates, and rice root behavior, which cannot be deduced from analysis of the bulk soil. N concentrations within fertiliser patches can be expected to be manifold higher than in the bulk soil It remains unanswered what scale and magnitude such differences might assume in waterlogged paddy soils and how the spatial variations of NH4+, NO3−, nitrification activity, pH, O2 and their temporal changes contribute to the spatial variation of the total inorganic N pool. The present study was designed to measure and analyze, by path analysis (see methods), soil NH4+, NO3−, pH, and O2 within N patches in paddy soils with the goal of substantially improving our understanding of the microbial nitrification process that is pivotal to rice N nutrition and of the factors that control it[33] within the heterogeneous reality of soil-N chemistry
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