Effects of herbicides on urea nitrogen transformation and greenhouse gas emission of soil in citrus orchards with different planting years

Soil microbes significantly influence the transformation and fate of nitrogen in soils by participating in the biology and biochemistry processes of soil nitrogen cycle. Research has shown that herbicides inhibit non-target soil microbes and the related biochemical processes. Therefore, herbicides have significant effects on plant uptake and utilization of nitrogen and its release from soil into the environment. This experiment aimed to explore the effects of herbicides on the transformation of urea nitrogen and greenhouse gas emissions in soils planted with citrus orchards for different years. It was important to evaluate the environmental safety of herbicides and nitrogen application in orchard soils. Thus a 2-factor, 3-level complete block design experimentwith herbicide factors of no herbicide, glyphosate of 10 mg·kg^-1 a.i. and butachlor of 10 mg·kg^-1 a.i.; and planting year factors of 0-year (woodland), 10-year and 30-year citrus orchard was conducted under laboratory conditions with 200 mg(N)·kg^-1(dry soil) urea applied in each treatment. The results showed that urea hydrolysis rate, nitrification rate, denitrification loss and greenhouse gas emission of citrus orchard (10 and 30 years) were higher than those of the woodland (P < 0.05). Compared with woodland, the 10-year and 30-year citrus orchards increased soil total denitrification loss by 5.12 and 4.30 times, total N₂O emission by 7.80 and 2.74 times, and total CO₂ emission by 19.62% and 39.64%, respectively. Soil urea hydrolysis rate, nitrification rate and CO₂ emission in the 30-year citrus orchard were significantly greater than those in the 10-year citrus orchard. Compared with the 10-year citrus orchard soil, total CO₂ emission increased by 16.74%, but total denitrification loss was no significantly different in the 30-year citrus orchard soil. Glyphosate and butachlor significantly boosted urea hydrolysis in woodland soil, but had no effect on nitrification in the 30-year citrus orchard soil. Butachlor had significant negative effect on woodland CO₂ emission, but showed no effect on citrus orchard soil. Compared with the zero herbicide treatment, butachlor significantly (P < 0.05) boosted N₂O emission, respectively by 56.27% and 85.41% in the 10-year and 30-year citrus orchard soils. Glyphosate had no effect on soil total CO₂ and N₂O emission in soils under the three treatments of citrus planting year. Herbicides glyphosate and butachlor had no significant effect on nitrogen transformation in citrus orchard soils, but butachlor significantly boosted N₂O emission in citrus orchard soils.