Study On Ammonia Deposition And Its Soil And Plant Responses In The Near-source Area Of Intensive Livestock Farm

Ammonia is an important alkaline gas in the atmosphere.Ammonia reacts with acid gas to survive NH₄NO₃,NH₄HSO₄ and（NH₄）₂SO₄,which are an important precursor substance of haze formation.Due to high protein feeding and low nitrogen use efficiency,farms are the main source of ammonia emissions,and ammonia emissions from animal husbandry contribute to 40%-50%of total global ammonia emissions.Due to the monsoon effect,getting rid of the drought and heat in the same latitudes of the world,and with the advantage of the climate with good water and heat,the hilly area of subtropical China plays an important role in livestock production in China.In the hilly region of subtropical China,the livestock production area and the natural area are staggered,and complex topography is not conducive to the diffusion and dilution of atmospheric pollutants.Ammonia emitted into the atmosphere from the farm has a short residence time in the atmosphere,is removed from the atmosphere by dry and wet deposition.Therefore,the near-source area of intensive farms becomes"hot spot"for ammonia deposition.Long-term high levels of ammonia deposition will have an adverse impact on natural and semi-natural ecosystems,such as soil acidification,changes in plant community structure,increased nitrate leaching,increased N₂O emission,etc.However,there are few studies of near-source ammonia deposition and its ecological effects in intensive farms in subtropical hilly areas.In this study,the intensive pig farm in Suizhou City,Hubei Province（with a monthly average stock of 8,900 heads）and the intensive dairy farm in Ningxiang City,Hunan Province（with a monthly stock of 800-1,000 heads）were selected as the research objects,and the experimental area was within 500 m of the farms.An empirical model was used to estimate ammonia emissions from pig farms,and based on the method of combining measured data and empirical models,the ammonia emissions from the dairy farm were estimated.The DELTA（DEnuder for Long-Term Atmospheric sampling）system and the ALPHA（Adapted Low Cost Passive High Absorption）system were used to monitor the ammonia concentration in near-source of the pig farm and the dairy farm for one year,respectively.The monitoring time of the pig farm was from July 2018 to June 2019,and the monitoring time of the dairy farm was from October 2019to September 2020.Ammonia deposition was estimated by a two-way exchange model.The relationship between ammonia deposition in the near-source area of the farms and the ammonia emissions from the farms was analyzed.A systematic study was carried out on the response characteristics of soil and plants to ammonia deposition in the near-source area of the farms.This study provides a basis for the prevention and control of ammonia pollution in the near-source area of intensive farms.The main findings are as follows:（1）Intensive farms emit large amounts of ammonia to the atmosphere during production.The ammonia emissions of the experimental farms were mainly in the animal houses,and the ammonia emissions had obvious seasonality,high in summer and low in winter.During the experiment,ammonia emissions from the pig farm were63000 kg N a^-1.The ammonia emission factor of fattening pigs was 6.4 kg N head^-1 a^-1.The daily ammonia concentration in the padding area of the dairy farm was 125-674μg N m^-3(average 356μg N m^-3),and the daily ammonia concentration in the scraping area of the dairy farm was 155-975μg N m^-3(average 503μg N m^-3).Ammonia emissions from the dairy farm were 26353 kg N a^-1.The ammonia emission factor of dairy cows was26.4 kg N head^-1 a^-1.（2）Ammonia deposition decreased with the increase of distance from the farms.Within 500 m of the experimental pig farm,the ammonia deposition was 5400 kg N a^-1,accounting for 8.6%of the ammonia emissions from the pig farm.Within 500 m of the dairy farm,the ammonia deposition was 4078 kg N a^-1,accounting for15.5%of the ammonia emissions from the dairy farm.The ammonia concentration decreased with the increase of the distance from the farms.The ammonia concentration decreased by about 85%,from 50-500 m to the pig farm.The ammonia concentration of 500 m away from the dairy farm was close to the background value.Average monthly ammonia concentrations at 50 m,100 m,200 m,300 m,and 500 m from the pig farm were 445μg N m^-3,320μg N m^-3,211μg N m^-3,143μg N m^-3,and 68μg N m^-3,respectively.Average monthly ammonia concentrations at 50 m,100 m,200 m,300 m,500 m from the dairy farm were 90.5μg N m^-3,38.3μg N m^-3,25.1μg N m^-3,7.0μg N m^-3,and 2.6μg N m^-3,respectively.（3）Continuous high-level ammonia deposition will lead to soil nitrogen enrichment and acidification,and affect plant available nitrogen status and plant community structure.The results showed that soil nitrate nitrogen content,soil ammonium nitrogen content,and soil p H in some transects near the source area of the farms had a regular trend of change with the increase of the distance to the farms.however,the change trend of other soil indicators was not obvious.This might be due to short operation time of the farms（no more than 10 years）,and there was high spatial variability of soil and plants in the experimental area.During the experiment,the soil inorganic nitrogen in the near-source area of the pig farm was dominated by ammonium nitrogen,while the soil inorganic nitrogen in the near-source area of the dairy farm was dominated by nitrate nitrogen.This might be due to the lower soil nitrification potential in the near-source area of the experimental pig farm.The content of soil inorganic nitrogen in the near-source areas of the pig farm and the dairy farm was 2.3-32.4 mg kg^-1(average 11.3mg kg^-1)and 2.4-73.0 mg kg^-1(average 30.6 mg kg^-1),respectively.The total nitrogen content of grass,bush,and moss leaves was 1.0%-2.4%（average 1.4%）,1.1%-2.4%（1.5%）,and 1.2%-2.6%（average 1.8%）,respectively.The leafδ¹⁵N values of grass,bush,and moss leaves were-10.3‰--4.1‰（average-7.9‰）,-12.6‰--2.3‰（average-8.3‰）,and-12‰--4.2‰（average-7.4‰）,respectively.In this study,the response of plant leaf total nitrogen content to ammonia deposition was not obvious.The reason might be that soil nitrogen was in a saturated state,and plants were not limited by nitrogen.δ¹⁵N can be used to identify nitrogen sources of plant leaves.Compared with total nitrogen content in plant leaves,δ¹⁵N in plant leaves has a better biological indicator for ammonia deposition.In the near-source area of the experimental pig farm,increasing ammonia deposition(from20 to 90 kg N hm^-2 a^-1)changed plant community structure.The percent cover of the grass specie Imperata cylindrica（L.）Beauv increased significantly（increased by about 46%）,but that of the bush specie Vitex negundo L decreased significantly（decreased by about 27%）.（4）The results of soil N₂O emission under simulated nitrogen addition showed that:adding nitrogen source treatment,during 35 days of incubation,the N₂O emission flux showed a trend of increasing first,then decreasing and finally increasing.There were significant differences in N₂O cumulative emissions from the soil around the farm under different nitrogen forms.During the experiment,soil ammonium nitrogen content decreased significantly,indicating that ammonia deposition in the near-source area of the farm will increase soil N₂O emissions through nitrification.