黄土丘陵区植被恢复对土壤可溶性氮组分的影响

为探究黄土丘陵地区人工植被恢复对土壤氮素养分累积与有效性的影响,研究分析了植被恢复15年刺槐、柠条、刺槐侧柏混交、刺槐山桃混交以及荒草地土壤可溶性氮组分含量及其垂直分布特征。结果表明,与耕地相比,植被恢复显著提高了0—30 cm土壤可溶性氮组分含量,这也使0—30 cm土壤可溶性氮组分密度显著增加,可溶性有机氮密度增幅表现为柠条(262.2%)刺槐(232.8%)刺槐山桃混交、刺槐侧柏混交(34.5%)荒草地(-21.5%),硝态氮密度整体表现为柠条刺槐刺槐山桃混交荒草地刺槐侧柏混交,增幅为7.9%—182.8%,铵态氮密度以刺槐山桃混交增幅最大(110.3%),荒草地最小为2.6%。可溶性有机氮、硝态氮占全氮的比例以刺槐最高,分别提升了2.4倍和0.6倍,铵态氮占全氮的比例以刺槐山桃混交最高,提升了1.0倍。可溶性氮组分受微生物量碳氮的影响大于有机质和全氮,微生物量氮与可溶性氮组分的相关性优于微生物量碳,硝态氮对土壤有机质、全氮和微生物量碳氮的变化最为敏感。综上,植被恢复能够提高土壤可溶性氮组分含量、密度及其占全氮比例,增加土壤氮的有效性,以刺槐、柠条提升效果最好。

Effects of vegetation restoration on soil soluble nitrogen in the Loess Hilly Region

Vegetation restoration has a significant impact on the physical and chemical properties of soil, and can improve soil organic carbon and total nitrogen. Soil soluble nitrogen pools have a major impact on the usually very small but rapidly cycling nitrogen pools such as NH₄⁺-N. Therefore, they will possibly play an important role in the nitrogen transformation pathways and plant uptake. However, the response of soil soluble nitrogen to different vegetation restoration patterns in the Loess Hilly Region of China is still poorly understood. We studied the distribution of soil soluble nitrogen in soil layers at depths of 0-30 cm under five conversion lands: Robinia pseudoacacia (RP), Caragana korshinskii (CK), R. pseudoacacia and Platycladus orientalis mixed forest (RP+PO), R. pseudoacacia and Prunus davidiana mixed forest (RP + PD), and abandoned farmland (AF), which have been converted from slope farmland (SF) for 15 years in the Loess Hilly Region. Contents of soil soluble organic nitrogen (SON), ammonium nitrogen (NH₄⁺-N), and nitrate nitrogen (NO₃^--N), and their densities and ratio to total nitrogen were determined to investigate the availability of soil nitrogen. Results show that the average contents of SON, NH₄⁺-N, and NO₃^--N of vegetation recovery lands was significantly higher than that of the slope farmland in the 0-30 cm soil layer (P < 0.05). Compared with other vegetation recovery lands, the SON and NO₃^--N improvement trend was better for RP and CK in the 0-30 cm soil layer, whereas RP + PD vegetation recovery lands exhibited the most NH₄⁺-N growth. After long-term vegetation recovery, the density of soil soluble nitrogen was also improved. Compared with SF, the SON rates increased in the 0-30 cm soil layer in the following order: CK (262.2%) > RP (232.8%) > RP + PD, RP + PO (average increase rate was 34.5%) > AF (-21.5%). The density of NO₃^--N in the 0-30 cm soil layer improved from 7.9 to 182.8% in the following order: CK > RP > RP + PD > AF > RP + PO. However, the maximum increase rate of NH₄⁺-N density was seen in RP + PD, whereas the minimum increase rate was seen in AF. Moreover, the ratio of SON and NO₃^--N to total nitrogen in RP was increased by 2.4 and 0.6, respectively, the ratio of NH₄⁺-N to total nitrogen in RP + PD has increased by 1, as compared to SF. The soil SON was not affected by soil depth, whereas the NO₃^--N and NH₄⁺-N content decreased with soil depth in the profiles. Further, based on correlation and regression analyses, we found that the soil soluble nitrogen was affected by soil organic matter, total nitrogen, and microbial biomass carbon and nitrogen. However, among these factors, microbial biomass was more sensitive than the others, and microbial nitrogen had a higher influence than microbial carbon. From another perspective, the NO₃^--N content was sensitive to changes in soil organic matter, total nitrogen, and microbial biomass carbon and nitrogen in long-term vegetation recovery. These findings demonstrate that vegetation recovery can improve the content, density, and proportion of soil soluble nitrogen, and increase the availability of soil nitrogen. It was also observed that converting SF to RP and CK forests is an efficient method of nitrogen activation in the Loess Hilly Region of China.