Spatial zoning of microbial functions and plant-soil nitrogen dynamics across a riparian area in an extensively grazed livestock system

Abstract

Anthropogenic activities have significantly altered global biogeochemical nitrogen (N) cycling leading to major environmental problems such as freshwater eutrophication, biodiversity loss and enhanced greenhouse gas emissions. The soils in the riparian interface between terrestrial and aquatic ecosystems may prevent excess N from entering freshwaters (e.g. via plant uptake, microbial transformations and denitrification). Although these processes are well documented in intensively managed agroecosystems, our understanding of riparian N removal in semi-natural systems remains poor. Our aim was to assess the spatial zoning of soil microbial communities (PLFA), N cycling gene abundance (archaeal and bacterial amoA, nifH, nirK, nirS, nosZ), N processing rates and plant N uptake across an extensively sheep grazed riparian area. As expected, soil properties differed greatly across the riparian transect, with significant decreases in organic matter, NH 4 + , carbon (C) and N content closest to the river (< 10 m). In addition, different microbial community structures were found along the transect. The abundance of N fixation (nifH) increased with distance from the river (> 10 m), while ammonia oxidising archaea (AOA) increased in abundance towards the river. N 2 O emissions rates were limited by C and to a lesser extent by N with greater emissions close to the river. Plant uptake of urea-derived 15 N was high (ca. 55-70% of that added to the soil) but 30-65% of the N was potentially lost by denitrification or leaching. Percentage recovered also suggests that the spatial patterning of plant and microbial N removal processes are different across the riparian zone. Our study provides novel insights into the underlying mechanisms controlling the spatial variability of N cycling in semi-natural riparian ecosystems.

Relationships between Soil Nutrients and Plant Diversity in Riparian Woodlands Along the Middle and Lower Reaches of the Yellow River, China

“…In contrast, studies conducted in natural or semi-natural habitat conditions such as grassland, forests, and woodlands, showed no significant changes between riparian zones and adjacent fields or upland forests [45,[46][47][48]. Moreover, some studies have indicated that soil nutrient content increased significantly with distance to the river [49][50][51]. Our own data support these findings when analyzing TC, TOC, TN, and A-P.…”

“…On the one hand, flood regimes impose stress and disturbance on riparian plants and soils near the river and redistribute sediment and organic matter. This can cause gradient distribution of plants and soils along the lateral distance of the riparian zone, according to flooding tolerance of riparian plants and deposition process of sediments, respectively [34,51]. On the other hand, riparian plant species diversity is higher in areas with groundwater discharge because of the higher nutrient availability [4,12].…”

Determining the Impact of Riparian Wetlands on Nutrient Cycling, Storage and Export in Permeable Agricultural Catchments

“…Whilst riparian wetlands have been shown to act as biogeochemical cycling hotspots, with the potential to transform and attenuate the nutrient and geochemistry signal from the catchment as it moves from land to water [1,3,[17][18][19], the efficacy of this function in real terms is controlled by the specific distribution of functional zones or biogeochemical cycling hotspots within the wetland. These in turn are controlled by the chemical character and distribution of contributing source areas of flow to the wetland between and within a water year, flow routing through the wetland, the residence time of water within the wetland and contact or exposure time between the microbial community and the nutrient parcel transitioning through the wetland; while soil organic carbon content and dissolved organic matter in porewaters acts as an energy resource to support denitrification, with redox status varying laterally, vertically and over time [7,11,20,21]. Reported denitrification rates vary widely within and between riparian wetlands, and over time, with annual rates reported to vary from <3 kg N ha −1 to >500 kg N ha −1 in denitrification hotspots in wet meadow systems, and <4 kg N ha −1 to >65 kg N ha −1 in riparian forests [22].…”

Soil microbes of an urban remnant riparian zone have greater potential for N removal than a degraded riparian zone

“…Many studies show that denitrification rates increase with increasing soil moisture (Burgin et al, 2010) and carbon concentrations (Groffman and Crawford, 2003;Xiong et al, 2017), root biomass (Gift et al, 2010) and plant species richness (Xiong et al, 2017;Ye et al, 2017), and positively correlate with increased abundance of key denitrification genes: nirS, nirK and nosZ (Tatariw et al, 2013;Rocca et al, 2015;Xiong et al, 2017;de Sosa et al, 2018b). However, other studies report that denitrification rates are not correlated with denitrifier abundance (Dandie et al, 2008;Henderson et al, 2010) and are highly variable depending on local conditions, i.e., season (Burgin et al, 2010;Tomasek et al, 2017).…”