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Increased nitrogen (N) concentrations in water bodies have highlighted issues regarding nutrient pollution in agricultural catchments. In this study, the ammonium-N (NH -N), nitrate-N (NO -N), and total N (TN) concentrations were observed in the stream water and groundwater of two contrasting catchments (named Tuojia and Jianshan) in subtropical central China from 2010 to 2014, to determine the rice agriculture impacts on the hydrographic patterns, and N export characteristics of the catchments. The results suggested that greater amounts of stream flow (523.0 vs. 434.7 mm year) and base flow (237.6 vs. 142.8 mm year) were produced in Tuojia than in Jianshan, and a greater base flow contribution to stream flow and higher frequencies of high-base flow days were observed during the fallow season than during the rice-growing season, indicating that intensive rice agriculture strongly influences the catchment hydrographic pattern. Rice agriculture resulted in moderate N pollution in the stream water and groundwater, particularly in Tuojia. Primarily, rice agriculture increased the NH -N concentration in the stream water; however, it increased the NO -N concentrations in the groundwater, suggesting that the different N species in the paddy fields migrated out of the catchments through distinct hydrological pathways. The average TN loading via stream flow and base flow was greater in Tuojia than in Jianshan (1.72 and 0.58 vs. 0.72 and 0.15 kg N ha month, respectively). Greater TN loading via stream flow was observed during the fallow season in Tuojia and during the rice-growing season in Jianshan, and these different results were most likely a result of the higher base flow contribution to TN loading (33.5 vs. 21.3%) and greater base flow enrichment ratio (1.062 vs. 0.876) in Tuojia than in Jianshan. Therefore, the impact of rice agriculture on catchment eco-hydrological processes should be considered when performing water quality protection and treatment in subtropical central China. [ABSTRACT FROM AUTHOR]

Increasingly, the characteristics of nitrogen (N) loading have been recognized to be critical for the maintenance and restoration of water quality in agricultural watersheds, in response to the spread of water eutrophication. This paper estimates N loading and investigates its influencing factors in ten small watersheds variously dominated by forest and agricultural land use types in the subtropics of China, over an observation period of 23-29 months. The results indicate that the average concentrations of total nitrogen (TN), NH-N, and NO-N were 0.83, 0.07, and 0.46 mg N L in the forest watersheds and 1.49-5.16, 0.21-3.23, and 0.99-1.30 mg N L in the agricultural watersheds, respectively. Such concentrations exceed the national criteria for nutrient pollution in surface waters considerably, suggesting severe stream pollution in the studied agricultural watersheds. The average annual TN loadings (ANL) were estimated to be 1,640.8 kg N km year in the agricultural watersheds, 63.3-86.1 % of which was composed of dissolved inorganic N (DIN; comprising NO-N and NH-N). The watershed with intensive livestock production (i.e., the maximum livestock density of 2.66 animal units (AU) ha) exhibited the highest ANL (2,928.7 kg N km year) related to N loss with effluent discharge. The results of correlation and principle component analysis suggest that livestock production was the dominant influencing factor for the TN and NH-N loadings and that the percentages of cropland in watersheds can significantly increase the NO-N loading in agricultural watersheds. Therefore, to restore and maintain water quality, animal production regulations and more careful planning of land use are necessary in the agricultural watersheds of subtropical China. [ABSTRACT FROM AUTHOR]

Land use has obvious influence on surface water quality; thus, it is important to understand the effects of land use patterns on surface water quality. This study explored the relationships between land use patterns and stream nutrient levels, including ammonium-N (NH-N), nitrate-N (NO-N), total N (TN), dissolved P (DP), and total P (TP) concentrations, in one forest and 12 agricultural catchments in subtropical central China. The results indicated that the TN concentrations ranged between 0.90 and 6.50 mg L and the TP concentrations ranged between 0.08 and 0.53 mg L, showing that moderate nutrient pollution occurred in the catchments. The proportional areal coverages of forests, paddy fields, tea fields, residential areas, and water had distinct effects on stream nutrient levels. Except for the forest, all studied land use types had a potential to increase stream nutrient levels in the catchments. The land use pattern indices at the landscape level were significantly correlated to N nutrients but rarely correlated to P nutrients in stream water, whereas the influence of the land use pattern indices at the class level on stream water quality differentiated among the land use types and nutrient species. Multiple regression analysis suggested that land use pattern indices at the class level, including patch density (PD), largest patch index (LPI), mean shape index (SHMN), and mean Euclidian nearest neighbor distance (ENNMN), played an intrinsic role in influencing stream nutrient quality, and these four indices explained 35.08 % of the variability of stream nutrient levels in the catchments ( p<0.001). Therefore, this research provides useful ideas and insights for land use planners and managers interested in controlling stream nutrient pollution in subtropical central China. [ABSTRACT FROM AUTHOR]

Landscape pattern can affect the export of non-point source nitrogen (N) in watersheds by regulating hydrological and biogeochemical processes. However, due to the inherent spatiotemporal variability of N exports under different land uses and hydrological regimes, it is essential to quantitatively evaluate the impact of landscape pattern on total N export coefficients (TNECs) and identify the threshold effect of TNECs between key landscape metrics. Based on river monitoring data for eight years in nine catchments of a typical agricultural watershed in the central subtropical region of China, we estimated the spatiotemporal variability and uncertainty of typical land-use TNECs (cropland, forest, and tea gardens) by combining an improved pollutant export coefficient model and Bayesian statistics. Subsequently, the contribution of landscape pattern to the variability in land use TNECs under different hydrological regimes was quantified, and the abrupt change points of TNECs along with the gradient of key landscape metrics were further determined. The results revealed that landscape pattern only had a significant effect on TNECs under medium-flow (representing discharge values within the 30–70th percentile range) and low-flow hydrological regimes (70–100th percentile range), explaining 48.1–54.7% and 55.2–69.3% of the variability, respectively. Threshold effects were observed between key landscape metrics representing different landscape categories (area edge, shape, and aggregation) and TNECs for each land use, and the threshold intervals corresponding to the TNEC abrupt change points were relatively consistent under medium- and low-flow regimes. These results serve as a critical reference for land-use planning and management to improve water quality, the approaches developed can be broadly extended to other regions with diverse land-use types and pollutant sources. [Display omitted] • Spatiotemporal variability of land use TNECs are addressed using Bayesian statistics. • Landscape pattern significantly affect TNECs under medium- and low-flow regimes. • The Landscape-TNEC relationship exhibits threshold-driven change. • The abrupt change analysis in landscape-TNECs is vital to landscape planning. [ABSTRACT FROM AUTHOR]

Base flow is recognized as an important hydrological pathway for NO 3 − –N export, however, the base flow contribution to NO 3 − –N loading in rice agriculture catchments remains unknown. In this study, stream discharge and NO 3 − –N concentration were observed in two contrasting rice agriculture catchments (named Tuojia and Jianshan) in subtropical central China between November 2010 and December 2013, to quantify the base flow contribution to NO 3 − –N loading and determine its relationship with rice agriculture. The results suggested that Tuojia produced more base flow (727.0 vs. 426.5 mm) and had higher base flow contribution to stream discharge (41.9% vs. 28.4%) than Jianshan did during the observation period, due to the more groundwater recharge associated with the higher areal proportion of rice agriculture in Tuojia. The average flow-weighted NO 3 − –N concentration in the base flow was higher in Tuojia than in Jianshan (1.43 vs. 1.07 mg N L −1 ), because rice agriculture could result in obvious N leaching into groundwater system. The NO 3 − –N loading via the base flow reached 0.27 kg N ha −1 month −1 in Tuojia, which contributed 36.5% of the NO 3 − –N loading via the stream discharge. These values were much greater than 0.12 kg N ha −1 month −1 and 27.3% in Jianshan. The more NO 3 − –N loading and greater base flow contribution in Tuojia were attributed to the more base flow and higher NO 3 − –N concentration in base flow associated with the intensive rice cropping. Specifically, the base flow contribution to the NO 3 − –N loading was greater during the fallow seasons than during the rice-growing seasons, likely due to the NO 3 − –N “landscape memory” effects from previous rice cropping seasons. Therefore, NO 3 − –N reduction practices in the rice agriculture catchments should be applied to mitigate the base flow contribution to NO 3 − –N loading in subtropical central China. [ABSTRACT FROM AUTHOR]

Highlights: [•] Rice agriculture affects nutrient chemical composition in stream water [•] Rice agriculture has a potential to degrade stream water quality [•] Influence of rice agriculture on nutrients in stream water is nonlinear [•] Influence is detectable when areal proportion of rice agriculture is >12–29% [Copyright &y& Elsevier]

• Landscape analyze needs to adopt a multidimensional perspective. • Landscape composition and configuration affects stream water N and P levels. • The most influential landscape composition and configuration for the N and P levels were identified. • The composition and configuration of landscape predicted stream water NP levels well. • Multidimensional landscape optimization benefits catchment environmental management. Although a landscape is a multidimensional integrated synthesis, the relationship between land use and water quality was merely studied in relation to protecting the aquatic ecological service function from the 'landscape perspective'. This study analyzed the integrated landscape pattern by coupling land use, soil property, and topography and determined the stream water total nitrogen (TN) and total phosphorus (TP) in Chinese subtropical catchments from 2010 to 2017 to reveal and quantify the relationship between multidimensional landscape patterns and stream water N and P levels using the GIS and partial least squares regression (PLSR) techniques. The results suggested that the areal proportions of tea field and residential area in the Ultisols and on the first three slope categories (0–28.02°) and paddy field in the Ultisols and on slope category I (0–5.35°) were positively correlated with the TN and TP, while forest in the Ultisols and on slope category IV (28.02–80.30°) was negatively correlated with the TN (p ≤ 0.05). For the multidimensional landscape metrics, the Euclidean nearest-neighbor distance (ENN_MN) and interspersion and juxtaposition index (IJI) were positively correlated with the TN, while the ENN_MN was positively correlated with the TP (p ≤ 0.05). The PLSR analysis explained the variations of TN and TP levels well (R2 ≥ 0.66), in which the areal proportions of the tea field and residential area in the Ultisols and on the first three slope categories (0–28.02°) and the paddy field in the Ultisols and on slope category I (0–5.35°) were the important variables for both TN and TP; however, the areal proportions of forest in the Ultisols and on slope category IV (28.02–80.30°) and the IJL were only important to the TN. Therefore, the multidimensional landscape pattern providing specific land use, soil property, and topography can be more capable in restoring and protecting stream N and P pollution at an applicable level. [ABSTRACT FROM AUTHOR]