Abstract
Determination of rates of mineralization of organic nitrogen (N) into ammonium-N (NH4+-N) and nitrification of NH4+-N into nitrate-N (NO3−-N) could be used to evaluate inorganic N supply capacity, which, in turn, could guide N fertilizer application practices in crop cultivation systems. However, little information is available on the change of mineralization and nitrification in soils under fruit cultivation systems converted from forestlands in karst regions. In a 15N-tracing study, inorganic N supply capacity in forest soils and three typical fruit crop soils under long-term cultivation was investigated, in addition to factors influencing the supply, in calcareous soils in the karst regions in southwestern China. Long-term fruit crop cultivation decreased soil organic carbon (SOC), total N, and calcium concentrations, cation exchange capacity (CEC), water holding capacity (WHC), pH, and sand content, significantly, but increased clay content. Compared to that of forests, long-term fruit crop cultivation significantly decreased mineralization and nitrification rates to 0.61–1.34 mg N kg−1 d−1 and 1.95–5.07 mg N kg−1 d−1, respectively, from 2.85–6.49 mg N kg−1 d−1 and 8.17–15.5 mg N kg−1 d−1, respectively, but greatly increased the mean residence times of NH4+-N and NO3−-N. The results indicate that long-term fruit crop cultivation could decrease soil inorganic N supply capacity and turnover in karst regions. Both mineralization and nitrification rates were significantly and positively correlated with SOC and total N concentrations, CEC, and WHC, but negatively correlated with clay content, suggesting that decreased soil organic matter and increased clay content were responsible for the decline in mineralization and nitrification rates in soils under long-term cultivation of fruit crops. The results of the present study highlight the importance of rational organic fertilizer application in accelerating soil inorganic N supply and turnover under long-term cultivation of fruit crops in karst regions.
Highlights
Karst geomorphology, which develops from carbonate rocks, is widely distributed on the Earth’s surface and China accounts for 15.6% of the karst region in the world, with the largest karst region (>1.9 million km2 ) found in southwestern China [1,2]
soil organic carbon (SOC), total N, and Ca concentrations, and cation exchange capacity (CEC), water holding capacity (WHC), and pH were significantly lower in soils under long-term fruit crop cultivations than in forest soils (p < 0.05), with similar trends observed at all three fruit crop sites examined (Table 1)
There were no significant differences in NH4 + -N concentrations between forest and fruit crop soils across the three study sites (p > 0.05), with considerable variations observed in NO3 − -N concentrations
Summary
Karst geomorphology, which develops from carbonate rocks, is widely distributed on the Earth’s surface and China accounts for 15.6% of the karst region in the world (more than 22 million km2 ), with the largest karst region (>1.9 million km2 ) found in southwestern China [1,2]. The population density is high and arable land resources become severely limited, so that people often deforest sloping lands in such regions to plant common crops (e.g., corn, soybean); the economic benefits of such cropping systems are extremely low. Such systems often induce negative effects in karst regions, such as large-scale “rocky desertification” [5,6,7]. High-yield and economically sustainable fruit crops (e.g., citrus or pitaya) have been cultivated extensively in the karst region in China to boost farmers’ incomes [3,7]. Investigating soil nutrient availability to evaluate soil fertility could guide fruit crop cultivation in lands converted from forests or woodlands in karst regions
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