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Abstract Extracellular glycosidases in soil, produced by microorganisms, act as major agents for decomposing labile soil organic carbon (e.g., cellulose). Soil extracellular glycosidases are significantly affected by nitrogen (N) fertilization but fertilization effects on spatial distributions of soil glycosidases have not been well addressed. Whether the effects of N fertilization vary with bioenergy crop species also remains unclear. Based on a 3-year fertilization experiment in Middle Tennessee, USA, a total of 288 soil samples in topsoil (0–15 cm) were collected from two 15 m 2 plots under three fertilization treatments in switchgrass (SG: Panicum virgatum L.) and gamagrass (GG: Tripsacum dactyloides L.) using a spatially explicit design. Four glycosidases, α-glucosidase ( AG ), β-glucosidase ( BG ), β-xylosidase ( BX ), cellobiohydrolase ( CBH ), and their sum associated with C acquisition ( C acq ) were quantified. The three fertilization treatments were no N input (NN), low N input (LN: 84 kg N ha −1  year −1 in urea) and high N input (HN: 168 kg N ha −1  year −1 in urea). The descriptive and geostatistical approaches were used to evaluate their central tendency and spatial heterogeneity. Results showed significant interactive effects of N fertilization and crop type on BX such that LN and HN significantly enhanced BX by 14% and 44% in SG, respectively. The significant effect of crop type was identified and glycosidase activities were 15–39% higher in GG than those in SG except AG . Within-plot variances of glycosidases appeared higher in SG than GG but little differed with N fertilization due to large plot-plot variation. Spatial patterns were generally more evident in LN or HN plots than NN plots for BG in SG and CBH in GG. This study suggested that N fertilization elevated central tendency and spatial heterogeneity of glycosidase activities in surficial soil horizons and these effects however varied with crop and enzyme types. Future studies need to focus on specific enzyme in certain bioenergy cropland soil when N fertilization effect is evaluated. 

Abstract BackgroundSoil moisture, pH, dissolved organic carbon and nitrogen (DOC, DON) are important soil biogeochemical properties in switchgrass (SG) and gamagrass (GG) croplands. Yet their spatiotemporal patterns under nitrogen (N) fertilization have not been studied. AimsThe objective of this study is to investigate the main and interactive effects of N fertilization and bioenergy crop type on central tendencies and spatial heterogeneity of soil moisture, pH, DOC and DON. MethodsBased on a 3‐year long fertilization experiment in Middle Tennessee, USA, 288 samples of top horizon soils (0–15 cm) under three fertilization treatments in SG and GG croplands were collected. The fertilization treatments were no N input (NN), low N input (LN: 84 kg N ha⁻¹in urea) and high N input (HN: 168 kg N ha⁻¹in urea). Soil moisture, pH, DOC and DON were quantified. And their within‐plot variations and spatial distributions were achieved via descriptive and geostatistical methods. ResultsRelative to NN, LN significantly increased DOC content in SG cropland. LN also elevated within‐plot spatial heterogeneity of soil moisture, pH, DOC and DON in both croplands though GG showed more evident spatial heterogeneity than SG. Despite the pronounced patterns described above, great plot to plot variations were also revealed in each treatment. ConclusionThis study informs the generally low sensitivity of spatiotemporal responses in soil biogeochemical features to fertilizer amendments in bioenergy croplands. However, the significantly positive responses of DOC under low fertilizer input informed the best practice of optimizing agricultural nutrient amendment. 

Abstract Nitrogen (N) fertilization significantly affects soil extracellular oxidases, agents responsible for decomposition of slow turnover and recalcitrant soil organic carbon (SOC; e.g., lignin), and consequently influences soil carbon sequestration capacity. However, it remains unclear how soil oxidases mediate SOC sequestration under N fertilization, and whether these effects co‐vary with plant type (e.g., bioenergy crop species). Using a spatially explicit design and intensive soil sampling strategy under three fertilization treatments in switchgrass (SG:Panicum virgatumL.) and gamagrass (GG:Tripsacum dactyloidesL.) croplands, we quantified the activities of polyphenolic oxidase (PHO), peroxidase (PER), and their sum associated with recalcitrant C acquisition (OX). The fertilization treatments included no N fertilizer input (NN), low N input (LN: 84 kg N ha⁻¹ year⁻¹in urea), and high N input (HN: 168 kg N ha⁻¹ year⁻¹in urea). Besides correlations between soil oxidases and SOC (formerly published), both descriptive and geostatistical approaches were applied to evaluate the effects of N fertilization and crop type on soil oxidases activities and their spatial distributions. Results showed significantly negative correlations between soil oxidase activities and SOC across all treatments. The negative relationship of soil oxidases and SOC was also evident under N fertilization. First, LN significantly depressed oxidases in both mean activities and spatial heterogeneity, which corresponded to increased SOC in SG (though by 5.4%). LN slightly influenced oxidases activities and their spatial heterogeneity, consistent with insignificant changes of SOC in GG. Second, HN showed trends of decrease in soil oxidase activities, which aligned with the significantly enhanced SOC in both croplands. Overall, this study demonstrated that soil oxidase activities acted as sensitive and negative mediators of SOC sequestration in bioenergy croplands and optimizing fertilizer use particularly in switchgrass cropland can improve for both carbon sequestration and environmental benefit.