Impact Of Land Use Change On Soil Nitrogen Accumulation And Loss In The Northern Slope Region Of Qinling Mountains

Substantial changes in land?use patterns have occurred in China since 1980s,mainly driven by economic development and rise in living standards of the people.Large areas of cereals have been converted to horticultural crops?e.g.,fruit trees and vegetables?due to their high economic value.Zhouzhi county and Meixian county are located on the northern slope region of Qinling Mountains and represent a model of dramatic land use change since the past 30 years,where large areas of cereals have been converted into kiwifruit orchards.This region is the largest kiwifruit production belt,accounting for 60%of the total kiwifruit planting area in China.However,over nitrogen?N?fertilization and improper irrigation in kiwifruit orchards are common practices leading to the waste of nutrients and water resources,and posing a huge risk of environmental pollution.Therefore,we selected a typical hilly catchment,that is Yujiahe catchment,located in Zhouzhi county,Shaanxi Province,China,to evaluate the status of N fertilization through field questionnaire surveying and status of nitrate accumulation in soil profiles by collecting soil samples in cereal croplands and kiwifruit orchards,and to study the spatial variation of soil nitrate accumulation in different soil layers and slope positions at field and catchment scales.At the same time,groundwater and surface water samples were collected from this catchment to evaluate the effect of different land use patterns on nitrate concentrations in water.Secondly,to understand the deep soil nitrate accumulation and its spatial variation at the regional scale,deep soil profiles?0-5 m and 0-10 m?were sampled in the intensive kiwifruit production belt on the northern slope region of Qinling Mountains.Whilst,groundwater samples were also collected to evaluate effect of intensive kiwifruit production on the levels of groundwater nitrate concentrations.Finally,the effect of reducing N fertilization in kiwifruit orchard on the fruit yield,quality,nutrient content and soil nitrate accumulation was evaluated with a long-term field experiment from 2013 to 2019.The main conclusions are as follows:?1?The results of five consecutive surveys from 2013 to 2017 showed that the N fertilization was severe in kiwifruit orchards with an average annual surplus of 1099 kg N/ha,which is 4 times higher than that in cereal croplands in Yujiahe catchment.Results of soil samples in 2016 showed that the average nitrate accumulation within a profile 0-400 cm of orchards was 3288 kg N/ha,which was 16-fold higher than that of cereal fields.More than77.5%nitrate in the soil profile?0-400 cm?of the orchards was below 100 cm.Spatial variation of nitrate accumulation in soil profiles was found at field and catchment scale.The accumulated nitrate in soil profiles at the downslope was approximately 2 times higher than that at the upslope in the same sloping orchards.The accumulated nitrate in soil profiles at the lowland zone of the catchment was higher than that of the upland zone of the catchment.The results indicated that land use change from cereal cropland to orchard significantly increased the nitrate accumulation in soil profiles.Excessive nitrate moves not only vertically downwards to deeper soil depth but also laterally into lower elevations at both field and catchment scales.?2?The measurement of soil water and nutrient contents in different slope positions of a slope kiwifruit orchard showed that the soil water content was intimately related to the irrigation and precipitation.In particular,the water content at 0-20 cm reacted quickly after irrigation and precipitation.Obvious variation in soil water content was found deeper than100 cm depth after heavy irrigation and precipitation.Variable changes of soil water content were found in different slope positions of orchard.The increase of water content in the downslope was significantly higher than that in the upper slope after heavy irrigation.Before the rainy season,the soil nitrate accumulation within 0-100 cm and 100-200 cm profile was1307 and 712 kg N/ha.In comparison,the accumulated nitrate in the two soil layers was 902and 1358 kg N/ha,respectively,after the rainy season.The nitrate accumulation of 0-200 cm soil profile in downslope position was significantly higher than that in the upslope position.And the difference of nitrate accumulation with 0-200 cm soil profile between the downslope position and upslope position increased after a rainy season.The above results indicated that the irrigation and precipitation were the major drivers of soil nitrate leaching.?3?The analysis of groundwater samples collected from different land use patterns in the Yujiahe catchment for three consecutive years?2015?2017?found that the averaged EC values and concentrations of Ca²⁺,Mg²⁺,Na⁺,Cl^-,HCO₃^-and NO₃^-in groundwater with different land use patterns followed the order of kiwifruit orchard>cereal cropland>natural forest land.The average nitrate concentration of shallow groundwater in natural forest land,cereal cropland and kiwifruit orchard and deep groundwater in kiwifruit orchard were 32.5,34.8,55.3 and 28.9 mg/L,respectively.The proportions of the shallow and deep groundwater samples exceeding the WHO standard?50 mg/L?in the region with kiwifruit orchard were 56.3%and 22.2%,respectively.The?¹⁵N of groundwater nitrate also increased from natural forest land to kiwifruit orchard.The results of isotope analysis showed that the nitrate in the groundwater with kiwifruit orchard was mainly from manure.Overall,the results suggested that the land use patterns have a significant impact on the nitrate concentration of groundwater.Intensive kiwifruit production has severely affected the quality of groundwater due to excessive N fertilizer application.?4?The analysis of N concentrations in surface water in the Yujiahe catchment?from January to October of 2018?showed that the average total nitrogen?TN?concentrations ranged from 2.67 to 7.79 mg/L from the upstream with natural forest land to the downstream with kiwifruit orchard.The average TN concentration ranged between 4.48 to 6.92 mg/L in the dry season,and 2.91 to 7.79 mg/L in the wet season.The TN concentrations of all samples exceeded the class V?1.5?2.0 mg/L for TN?of Environmental Quality Standards of Surface Water of China?GB3838-2002?.The average concentrations of nitrate ranged from 2.15 to 7.11 mg/L?3.48 to 5.99 mg/L in dry season and 2.23 to 7.10 mg/L in wet season?.Nitrate concentrations were significantly higher than that of ammonium,accounting for 71.5%?91.5%of TN.Clear variation in nitrate concentrations of water samples was found in different land use patterns.The average nitrate concentration in the downstream area with intensive kiwifruit orchards was significantly higher than that in the upstream with natural forest land?p<0.05?.Compared with the dry season,the TN and nitrate concentrations of samples in upstream area significantly decreased in the wet season,while significantly increased in the downstream area.The analysis of isotopic composition of nitrate(?¹⁵N-NO₃^-and?¹⁸O-NO₃^-)in water samples showed that the nitrate of samples in the upstream area was mainly derived from the ammonium in atmospheric deposition,and for the nitrate from the area in the downstream was mainly derived from the application of ammonium fertilization and ammonium in atmospheric deposition.?5?The result of survey in the kiwifruit production belt on the northern slope region of Qinling Mountains showed that the annual average N input and surplus of kiwifruit orchards were 1332 kg/ha and 1206 kg/ha,which were significantly higher than those of cereal cropland?452 kg/ha and 252 kg/ha?.The average nitrate accumulation in 0-5 m soil profile of kiwifruit orchards was 3674 kg/ha,of which 84%was located in the soil layer below 1 m.In comparison,the nitrate accumulation in 0-2 m soil profile of cereal cropland was 158kg/ha,of which was mainly distributed in 0-1 m soil layer.The total nitrate accumulation of0-5 m and 0-10 m soil profiles in this kiwifruit production belt was 156.5 and 266.5 Gg N,respectively.The nitrate concentrations of more than 97%collected groundwater samples were over the WHO standard of 50 mg/L.Therefore,land use change from cereals to kiwifruit orchard in this region significantly increased the accumulation and leaching of soil nitrate,and posed a serious threat to the groundwater quality.?6?The long-term field experiment?2012?2019?results showed that compared with the farmers'conventional N fertilization,no adverse impact on fruit yield,quality and N content of fruit and branch was found when N fertilizer rate was reduced by 25%in 2012?2014 and by 45%in 2014?2019?p>0.05?,rather increased farmers'economic benefits and significantly reduced nitrate accumulation in 0-200 cm soil profile?p<0.05?.Compared with the common urea,application of controlled-release urea significantly reduced nitrate leaching to the deeper soil layers.Thus,there is a great potential for reducing N fertilization in kiwifruit orchards in this region.The nitrate accumulation in soils can be significantly reduced by decreasing the amount of N fertilization.In addition,the use of controlled-release fertilizers is worthy of being adopted at wider scale in the region.In conclusion,the land use change from cereal cropland to kiwifruit orchard on the northern slope region of Qinling Mountains significantly increased soil N surplus and nitrate accumulation.Improper irrigation and heavy rainfall were the main drivers of N leaching to the deeper soil.Nitrate leaching significantly contaminated the local surface and groundwater quality.Therefore,it is urgent to optimize N application rate in kiwifruit orchard and use the variable fertilization in the slope orchards for reducing nitrate accumulation and leaching in this region.At the same time,optimized irrigation strategies?e.g.,fertigation?also should be adopted to improve water use efficiency and decrease nitrate leaching.These strategies are of great significance to ensure sustainable and healthy development of the kiwifruit production in this region.