Abstract
Globally only ≈50% of applied nitrogen (N) fertilizer is captured by crops, and the remainder can cause pollution via runoff and gaseous emissions. Synchronizing soil N supply and crop demand will address this problem, however current soil analysis methods provide little insight into delivery and acquisition of N forms by roots. We used microdialysis, a novel technique for in situ quantification of soil nutrient fluxes, to measure N fluxes in sugarcane cropping soils receiving different fertilizer regimes, and compare these with N uptake capacities of sugarcane roots. We show that in fertilized sugarcane soils, fluxes of inorganic N exceed the uptake capacities of sugarcane roots by several orders of magnitude. Contrary, fluxes of organic N closely matched roots’ uptake capacity. These results indicate root uptake capacity constrains plant acquisition of inorganic N. This mismatch between soil N supply and root N uptake capacity is a likely key driver for low N efficiency in the studied crop system. Our results also suggest that (i) the relative contribution of inorganic N for plant nutrition may be overestimated when relying on soil extracts as indicators for root-available N, and (ii) organic N may contribute more to crop N supply than is currently assumed.
Highlights
Only ≈50% of applied nitrogen (N) fertilizer is captured by crops, and the remainder can cause pollution via runoff and gaseous emissions
While the small size of microdialysis probes means that a smaller volume of soil can be sampled, relative to that sampled via soil extracts, it allows the study of N dynamics in soil microsites
Fluxes of amino acids were more similar across soils than fluxes of inorganic N and significantly (P < 0 .003) higher in organic-fertilized soil (19 nmol N cm−2 h−1) than in urea-fertilized and unfertilized soils (13 and 14 nmol N cm−2 h−1, respectively; Fig. 1, Supplementary Table 1)
Summary
Only ≈50% of applied nitrogen (N) fertilizer is captured by crops, and the remainder can cause pollution via runoff and gaseous emissions. Fluxes of organic N closely matched roots’ uptake capacity These results indicate root uptake capacity constrains plant acquisition of inorganic N. Destructive soil sampling and subsequent processing introduces artefacts such that the size and composition of the N pools deviate from those in situ[17,18], suggesting that the widespread use of soil extracts as proxy for N availability to plants may be flawed. This conflict is illustrated when comparing information obtained with conventional excavation-sieving-extraction techniques and low-disturbance in situ microdialysis[19]. Matching soil N supply to the crops’ N demand is a key objective for improving the nutrient use efficiency of cropping systems[5,25,26,27]
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