Abstract
The possibility that N fertilizer increases soil organic matter (SOM) mineralization and, as a result, reduces SOM stocks has led to a great debate about the long-term sustainability of maize-based agroecosystems as well as the best method to estimate fertilizer N use efficiency (FNUE). Much of this debate is because synthetic N fertilizer can positively or negatively affect SOM mineralization via several direct and indirect pathways. Here, we test a series of hypotheses to determine the direction, magnitude, and mechanism of N fertilizer effect on SOM mineralization and discuss the implications for methods to estimate FNUE. We measured the effect of synthetic N fertilizer on SOM mineralization via gross ammonification at two long-term experiments in central and southern Iowa, USA with replicated plots of continuous maize that received one of three âhistoricalâ N fertilizer rates (zero, moderate or high) from 1999-2014. In 2015, prior to our measurements, we split the historical N fertilizer rate plots into two subplots that received either the site-specific agronomic optimum N rate or zero N fertilizer. At the onset of rapid maize N uptake, N fertilizer reduced gross ammonification by 13-21% (2-5 kg NH4-N ha-1 d-1). A companion laboratory experiment rejected the hypothesis that differences in net primary productivity between fertilized and unfertilized treatments explained the negative effect of N fertilizer on SOM mineralization. Moreover, the NH4+ pool size was negatively correlated with the gross ammonification rate (r2 = 0.85, p < 0.001). Thus, we conclude that NH4+-N fertilizer had a direct suppressive effect on SOM mineralization. These results demonstrate that the direct effect of N fertilizer on microbial activity can exceed the indirect effects of N fertilizer via large changes in NPP that alter organic matter inputs, soil temperature and moisture content. The magnitude of this effect and specificity to NH4+-N has significant implications for fertilizer management as well as the measurement and modeling of agroecosystem N dynamics including FNUE.
Highlights
Efficient use of nitrogen (N) fertilizer is important for economic crop production, environmental quality, and human health (Cassman, 1999)
Because we found no association between NOâ3 pool size and gross ammonification in field-fresh or laboratory-controlled soils, N fertilizer with large fractions of NOâ3 may have a lesser effect on gross ammonification
Synthetic N fertilizer addition consistently reduced gross ammonification at an early growth stage of maize when soil inorganic N concentrations were higher in N-fertilized treatments due to a direct inhibitory effect of NH+4 on microbial activity (Figure 8)
Summary
Efficient use of nitrogen (N) fertilizer is important for economic crop production, environmental quality, and human health (Cassman, 1999). Some reports indicate that synthetic N fertilizer reduces SOM stocks because it enhances soil organic matter (SOM) mineralization (Mulvaney et al, 2009; Russell et al, 2009); other reports indicate that it increases SOM stocks, at the agronomically optimum N fertilizer rate (AONR) because it increases net primary productivity (NPP; Poffenbarger et al, 2017). Much of this debate occurs because synthetic N fertilizer can positively or negatively affect SOM mineralization via several direct and indirect pathways (Table 1). An increase in NPP, including rhizodeposition, can increase SOM mineralization by increasing microbial biomass or enzyme activity and this effect is often referred to as âpositive primingâ (Cheng, 2009; Chen et al, 2014)
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