Abstract
Nitrogen (N) fertilizers can potentially alter spatial distribution of soil organic carbon (SOC) and total nitrogen (TN) concentrations in croplands such as switchgrass (SG: Panicum virgatum L.) and gamagrass (GG: Tripsacum dactyloides L.), but it remains unclear whether these effects are the same between crops and under different rates of fertilization. 13C and 15N are two important proxy measures of soil biogeochemistry, but they were rarely examined as to their spatial distributions in soil. Based on a three-year long fertilization experiment in Middle Tennessee, USA, the top mineral horizon soils (0–15 cm) were collected using a spatially explicit design within two 15-m2 plots under three fertilization treatments in SG and GG croplands. A total of 288 samples were collected based on 12 plots and 24 samples in each plot. The fertilization treatments were no N input (NN), low N input (LN: 84 kg N ha−1 in urea) and high N input (HN: 168 kg N ha−1 in urea). The SOC, TN, SOC/TN (C: N), δ13C and δ15N were quantified and their within-plot variations and spatial distributions were achieved via descriptive and geostatistical methods. Results showed that SG generally displayed 10~120% higher plot-level variations in all variables than GG, and the plot-level variations were 20~77% higher in NN plots than LN and HN plots in SG but they were comparable in unfertilized and fertilized plots in GG. Relative to NN, LN and HN showed more significant surface trends and spatial structures in SOC and TN in both croplands, and the fertilization effect appeared more pronounced in SG. Spatial patterns in C: N, δ13C and δ15N were comparable among different fertilization treatments in both croplands. The descending within-plot variations were also identified among variables (SOC > TN > δ15N > C: N > δ13C). This study demonstrated that N fertilizations generally reduced the plot-level variance and simultaneously re-established spatial structures of SOC and TN in bioenergy croplands, which little varied with fertilization rate but was more responsive in switchgrass cropland.
Highlights
A limited number of studies have focused on the spatial distributions of soil C and N in bioenergy croplands
We hypothesize that relative to those soils that have not been fertilized for 3 years, continued N fertilization at either low or high rates (84 and 168 kg N ha−1) will reduce the within-plot variances but restructure the spatial patterns of soil organic carbon (SOC), total nitrogen (TN), C:N, δ13C, and δ15N; these aforementioned fertilization effects little depend upon fertilization rate or crop type
The concentrated ranges of SOC and TN were shifted to greater values under high N input (HN) and low N input (LN) relative to N input (NN), but substantial overlaps between fertilization treatments were identified for other variables (Fig. S1)
Summary
A limited number of studies have focused on the spatial distributions of soil C and N in bioenergy croplands. Because N fertilization impacts on crop productivity and microbial activities occurred annually or over even a shorter time scale, this may likely shift spatial distribution and structure of soil C and N over month to year time scales. We speculate that the N fertilizer induced greater input to soil via litterfall and root exudate may have contributed to altered spatial variations of microbial biomass as observed formerly[22], and this can further stimulate microbial mineralization of more 13C enriched and older soil organic matter[35,36]. We hypothesize that relative to those soils that have not been fertilized for 3 years, continued N fertilization at either low or high rates (84 and 168 kg N ha−1) will reduce the within-plot variances but restructure the spatial patterns of SOC, TN, C:N, δ13C, and δ15N; these aforementioned fertilization effects little depend upon fertilization rate or crop type
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