| Nitrogen?N?is ubiquitous in ecosystems and essential to all living beings.N cycle is one of the key processes in controlling the compositions,diversities,dynamics and functions of terrestrial,limnetic and marine ecosystems.However,the N input by humans has surpassed that by natural process due to the intensification of anthropogenic activities,thus,justifying its role in which human capabilities surpass natural forces.Great changes in land use,especially large-scale earth's surface reconstruction projects,have exerted unprecedented and considerable influences on N cycle of ecosystems.In order to quantify the effects of large-scale human projects on ecosystem N cycle,Gully Land Consolidation?GLC?project in the northern Loess Plaeatu?Yan'an City,Shaanxi Provience?was selected as an example to reveal the effects of large-scale human engineering activities,with investigating distribution of soil N,characteristics of water and N stable isotopes and water chemisty,spatial-temporal dynamics of water inorganic N in both treated and untreated watersheds.We discussed possible reasons behind those differences to explore the effects and mechanisms of GLC project on N cycle,and to determine the hot spots and hot moments that lead to the significant differences in N cycle in those watersheds.The main conclusions are as follow:?1?The spatial distribution patterns of soil nitrate?NO3-?,ammonium?NH4+?,and total nitrogen?TN?differed.Land cover,soil moisture and topographical factors showed remarkable effects on the spatial variations of soil N.Land cover,soil moisture,elevation,plan curvature and flow accumulation were identified as the dominant factors affecting the spatial variations of soil NO3-?p<0.05?,and land cover and slope aspect also controlled the changes in soil NH4+and TN in the studied catchment?p<0.001?.The areas with values greater than 14.65 kg ha-1,9.58 kg ha-1and 1.89 Mg ha-1 were considered the potential hotspots for soil NN,AN and TN,respectively.In the studied catchment,forestland,gully land and grassland were considered as the potential hotspots of soil NO3-,NH4+and TN accumulation,respectively.Regarding the slope aspect,the north-facing slopes were the potential hotspots of soil TN accumulation.?2?Different land use in the treated and untreated catchments showed different vertical distribution patterns of soil NO3-,NH4+and TN.In the two catchments,the topsoil in the forestland and grassland showed significantly higher NO3-and TN concentration than the subsoil?p<0.05?,and showed a pattern of shallow accumulation due to ecosystem N cycling.Due to the superior hydrological condition in the gully land,however,the vertical distribution patterns of soil NO3-,NH4+and TN in the gully land of the two catchments significantly differed from those in the forestland and grassland.In addition,in the treated catchment with gully land consolidation,soil NO3-concentration(2.05±0.62 mg L-1)in the 0-20 cm soil layer was lower than that(4.93±1.45 mg L-1)in the 80-100 cm soil layer,which showed a significant deep accumulation pattern in the created gully cropland due to hydrological leaching;while in the untreated catchment,soil NH4+concentration(5.16±0.87 mg L-1)in the 0-20 cm was lower than that(8.15±2.22 mg L-1)I n the80-100 cm soil layer,which showed a significant deep accumulation pattern in the gully area likely being the relatively higher ammonification rate.?3?The stable isotope rang of nitrate in the two watersheds were 0.9‰to 15.4‰for nitrogen,and-0.2‰to 27.1‰for oxygen,indicating that nitrate fertilizer,soil ammonium,and manure and septic waste were the main sources of surface water nitrate.While in the groundwater,the stable isotope rang of nitrate in the two watersheds were 0.63‰to 17.9‰for nitrogen,and-4.7‰to 7.8‰for oxygen,indicating that nitrate mainly comes from soil nitrification of ammonium,and manure and septic waste.Compared to the untreated watershed?the mean istopic value of surface water nitrate was 6.6±5.8‰?,the treated watershed?the mean istopic value of surface nitrate was 12.4±7.2‰?showed more input of nitrate fertilizers in the surface water.However,groundwater showed no significant difference in nitrate sources between the two watersheds.Results also demonstrated that the groundwater probably occurred denitrification and dissimilatory nitrate reduction to ammonium?DNRA?during the wet season,while during the dry and snowmelt seasons,less nitrification happened.The multivariate statistical analysis showed that the treated watershed had strong interactions between the surface runoff and the reservoir water,while in the untreated watershed,the streamflow had a close relationship with the groundwater.We concluded that valley reshaping and damming significantly affected the nitrate sources and water quality of the surface water,while showed little influence on the groundwater.However,the time lag between land consolidation and groundwater quality response should be paid attention in future.?4?The effects of GLC on inorganic N in the groundwater was not significant,however,the effects of GLC on the inorganic N in the surface water was significant,and the difference was mainly from the reservoirs.The concentrations of ammonia(0.11 mg L-1)of the reservoirs in the treated watershed were significantly lower than that(0.32 mg L-1)of the surface runoff in the untreated watershed during the dry season.The concentrations of nitrate of the reservoirs(wet season:0.41 mg L-1;dry season:0.52 mg L-1;normal season:0.68 mg L-1)in the treated watershed were remarkably lower than that of the surface runoff(wet season:1.19 mg L-1;dry season:1.25 mg L-1;normal season:0.88 mg L-1)in the untreated watershed throughout the studied period.In addition,compared with the untreated watershed,the ammonia of the surface water in the treated watershed is less affected by the domestic sewage and livestock excreta.?5?Precipitation showed a significant impact on the dynamics of ammonia in the reservoirs.At the weekly time scale,the concentrations of ammonia in the reservoirs were significantly increased immediately after few rainfall events;regarding other rainfall events,the increasing of ammonia concentrations in the reservoirs had certain lags relative to the occurrence of the precipitation.The changes of ammonia concentrations in the reservoirs lagged the rainfall events by 1-2 weeks at the weekly time scale and lagged the rainfall events by 1 month at the monthly time scale.The fluctuations of ammonia concentration in the reservoirs at the time of summer and autumn were more intense than those in winter and spring.In addition,the average concentrations of ammonia in the reservoirs at the time of summer(0.27mg L-1)and autumn(0.26 mg L-1)were higher than those in winter(0.09 mg L-1)and spring(0.11 mg L-1).The results showed that precipitation significantly affected the N cycle of the reservoir through the effects on ammonia in the surface runoff and dissolved oxygen level in the treated watershed.In summary,the new croplands and reservoirs were considered as the hot spots which significantly influence the watershed N cycle,and therefore considered as the key subproject in the treated watershed.In addition,the rainfall events were considered as the hot moments of N migration and transformation in the small watershed. |
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