The Influence of Land Use and Fish Farming on the Contents of Ca²⁺, Mg²⁺, K⁺and Na⁺in Pond and Reservoir Ecosystems in an Agricultural Small Watershed, China

It is not understood well that how the effects of land use and fish farming on the contents of alkali metals (Ca2+, Mg2+, K+ and Na+) in small water bodies of pond and reservoir ecosystems at the watershed scale. In this study, the concentrations of Ca2+, Mg2+, K+ and Na+ in water bodies were measured for 103 ponds and reservoirs used as fish farming or surrounded by different agricultural land use types in the subtropical hilly watershed of Jinjing (105 km2), China. The two important environmental factors of fishing farming and agricultural land use influenced the spatial variation the contents of alkali metals. The ponds and reservoirs in residential area had significantly higher concentrations of Ca2+ and Na+ than those with other land use types, reflecting the influence of domestic wastewater. Compared with those of natural ponds with non-fish farming, no significant increase of alkali metal contents occurred in fish farming ponds, due to the regular cleaning of ponds by farmers. However, the effect of fish culture on alkali metal contents was still supported indirectly by the fact that the alkali metal contents significantly correlated with nitrate contents in fish farming ponds and but high related with that of DIP in natural ponds. The suitability assessment for irrigation on the pond water indicated that almost all of ponds were suitable for irrigation except some ponds surrounded by residential area and tea plantation. Generally, our results demonstrated that fish farming and agricultural land use affected the contents of alkali metals in ponds and reservoirs. The agricultural water irrigation would be with caution from the ponds with tea plantation and residential area in the subtropical hilly watershed.

Determining nitrogen fate by hydrological pathways and impact on carbonate weathering in an agricultural karst watershed

Identifying the nitrogen(N)fate is complicated and a great challenge in karst watersheds because of the co-existence of natural pools and anthropogenic sources.The objective of the study was to use stable isotopic composition of dual-isotope(δ^(15)NNitrate and δ^(18)O_(Nitrate))and LOADEST model approaches to trace N sources,pathways in karst watershed.The study was conducted in the Houzhai watershed,which is a typical agricultural karst watershed from July 2016 to August 2018,to reveal the N fate and the coupled carbon(C)-N processes occurring in the riverine-watershed with agricultural activities.We found that the wet deposition of total nitrogen(TN)flux was 33.50 kg hm^(-2)·a^(-1)and dissolved nitrogen(DN)flux was 21.66 kg hm^(-2)·a^(-1).The DN runoff loss was 2.10×10^(5)kg·a^(-1)and the loss of DN during the wet season accounted for 95.4%over a year.In the wet season,NO_(3)^(-)-N daily efflux was 977.62±516.66 kg ha^(-1)·day^(-1)and 248.77±57.83 kg ha^(-1)·day^(-1)in the dry season.The NH_(4)^(+)-N efflux was 29.17±10.50 kg ha^(-1)·day^(-1)and 4.42±3.07 kg ha^(-1)·day^(-1)in the wet and dry seasons,respectively.The main form output load of N was NO_(3)^(-)-N which was more than 30 times as much as NH_(4)^(+)-N output loss.The NO_(3)^(-)N caused by rainfall contributed11.82%-53.61%to the export load.Nitrate from soil contributed over 94%of the N to Houzhai river caused by N leaching.In addition,manure and farmland soil were the main sources of groundwater in the Houzhai watersheds,the contribution rates were 25.9%and 22.5%.The chemical N fertilizers affected carbonate weathering strongly,and the HCO_(3)^(-) flux caused by nitrifi-cation due to N fertilizers application in soil accounted for 23.5%of the entire watershed.This study suggested that carbonate weathering may be influenced by nitrogen nitrification in the karst watershed.

Grid-cell based assessment of soil erosion potential for identification of critical erosion prone areas using USLE,GIS and remote sensing:A case study in the Kapgari watershed,India

Estimation of soil erosion is of paramount importance due to its serious environmental and societal concern.Soil erosion would have impact on fertility of agricultural land and quality of water.The major objective of this study was to investigate the spatial heterogeneity of annual soil erosion on the grid-cell basis in a small agricultural watershed of eastern India.The study watershed has a drainage area of 973 ha and is subdivided into three sub-watersheds namely:KGSW1,KGSW2 and KGSW3,based on the land topography and drainage network.Average annual soil erosion was estimated on 100 m×100 m grid-cells by integrating universal soil loss equation(USLE)model with GIS for subsequent identification of critical erosion prone areas.It was found that 82.63%area of the total watershed falls under slight-erosion-class(0–5 t-ha^(−1)-yr^(−1)),6.87%area lies under the moderate-erosion-class(5–10 t-ha^(−1)-yr^(−1)),5.96%area is under high-erosion-class(10–20 t-ha^(−1)-yr^(−1)),3.3%area of watershed lies under the very-high-erosion-class(20–40 t-ha^(−1)-yr^(−1))and 1.24%area falls under“severe-erosion-class”(40–80 t-ha^(−1)-yr^(−1)).The study revealed that the sub-watershed KGSW3 is critical due to the presence of the highest number of critical erosion prone grid-cells.The sediment delivery ratio(SDR)was also estimated to analyze the contribution of sediment yield at the sub-watershed level.Lowest SDR for the whole watershed as compared to sub-watersheds indicates that most of the eroded soil got deposited in rice crop check-basins before reaching the outlet.The reported results can be used for prioritizing critical erosion prone areas and for determining appropriate soil erosion prevention and control measures.