黄土高原区不同植物凋落物搭配对土壤微生物量碳、氮的影响

Effects of mixed plant residues from the Loess Plateau on microbial biomass carbon and nitrogen in soil

黄土高原丘陵沟壑区进行的以退耕还林还草为主的生态建设,使得进入土壤生态系统有机物的种类和数量发生变化,不同种类凋落物混合对土壤微生物量碳、氮的影响是值得关注的问题。采用室内培养试验方法研究了采自黄土高原地区6种不同植物凋落物及等比例混合后对土壤微生物量碳、氮及矿质态氮含量的影响。结果表明,加入不同植物凋落物均显著提高了培养期间土壤微生物量碳、氮含量。总体平均,添加3种等量混合后植物凋落物的土壤微生物量碳、氮含量高于两种凋落物等量混合处理,而两种凋落物混合高于单种凋落物处理;土壤矿质态氮含量的变化则相反,即单种>两种混合>3种混合。单种和两种混合后土壤微生物量碳、氮含量与其碳氮比显著相关,而3种凋落物混合后土壤微生物量碳、氮含量与其碳氮比无相关性,说明多种凋落物混合后土壤微生物量碳、氮含量受多种因素共同影响。因此,在黄土高原植被恢复重建中,有必要采用不同种类植物搭配,利用生物多样性促进生态系统健康持续发展。

Both the quality and quantity of organic materials from plant residues returned into the soil ecosystems in the gully and valley region of the Loess Plateau have substantially changed after the returning of croplands to forests and grasslands since 2000 in Northwestern China.However,no study has reported if soil carbon（C） or nitrogen（N） has been changed after one decade of plant residues returning to the soil.An important mechanism to understand these changes is to know if and how the returning of plant residues to the soil could alter soil microbial biomass C and N（SMBC,SMBN） and mineral N in this region.We conducted a time course（0 to 84 days） incubation experiment to study the effects of external addition of plant residues either from a single plant species,a mixture of two or three plant species on the soil microbial biomass carbon and nitrogen（SMBC,SMBN） and mineral N content under（25±3） ℃,70% field water holding capacity,dark and aerobic laboratory conditions.Aboveground plant residues from six local plant species（Hippophae rhamnoides Linn.,Medicago sativa Linn.,Populus simonii Carr.,Robinia pseudoacacia L.,Salix psammophila.and Stipa bungeana Trin.）,which had C to N ratios from 17.52 to 57.04,were taken from the Loess Plateau,Shaanxi,China.The soil used was a Loess soil（similar to Ustochnept in the US or Calcic cambisol in FAO system）.Equal amount of plant residues,as either a single plant species,a mixture of any two or three plant species were added to the soil for a total of 27 treatments including the non-plant-residue addition as the control.As a general rule,our results showed that the addition of either one single or two or three mixed plant residues to the soil significantly increased the contents of SMBC and SMBN.Overall,both the averaged SMBC and SMBN during incubation were highest in the treatment with the three-plant-species residues,higher in the treatment with the two-plant-species residues,and lowest in the treatment with the single plant species residue.In contrast to the SMBC and SMBN,contents of mineral nitrogen in the soil showed a reverse pattern among the three residue treatments： the three-plant-species residue the two-plant-species residue the single plant species residue addition.Meanwhile,there was a significantly positive relationship between the SMBC and/or SMBN and the C/N ratio in the one-plant-species or the two-species residues,but not in the three-plant-species residue.These results might indicate that SMBC and SMBN contents could have been affected by a range of factors,including incubation conditions（temperature,soil water holding capacity,incubation time period,etc.） and the chemical properties（C and N content,organic compounds,etc.） of the residue itself,particularly when the mixed three-plant-species residues were added to the soil.Our study suggested that both SMBC and SMBN contents could be substantially increased by returning of two or more than two-plant-species residues into soils,which could then contribute to the enhancement of vegetation restoration and soil fertility in the Loess Plateau.