黄土旱塬农田土壤有机碳、氮的演变与模拟
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摘要
土壤有机质是影响土地生产力的重要因素。有机质在改善土壤结构、保持水分、防止土壤侵蚀和退化、提供植物养分特别是氮素等方面具有重要作用。黄土高原是我国主要生态脆弱区，了解这一地区管理措施对有机质周转影响，分析该地区土地生产潜力是调控土壤有机质含量的基础。本文以中国科学院长武试验站的长期试验(始于1985年)为依托，通过对典型处理8-15年的土壤养分、作物产量以及同期降水量等的系统分析，揭示了黄土旱塬区不同种植体系中土壤有机碳、氮的演变规律：阐明了长期施肥条件下土壤硝态氮的迁移积累机理；提出了目前水肥条件下黄土旱塬区冬小麦生产潜力及其提高途径。在获得上述实验资料的基础上，建立了黄土旱塬区土壤有机碳周转的模拟模型。其主要新进展如下：
(1)在黄土旱塬区，禾本科连作能够保持土壤有机碳、氮的含量(6.50gkg~(-1)、0.82gkg~(-1)；禾本科与豆科作物轮作土壤有机碳、氮积累略有提高；小麦与红豆草轮作或苜蓿连作对提高土壤有机碳、氮积累具有显著的促进作用；长期休闲土壤有机碳、氮呈缓慢降低趋势。
(2)不同作物系统中，增施化肥显著提高了土壤有机碳、氮的积累，有机碳、氮增幅分别为19-33％、22-35％。
(3)本区降水量的不足与多变限制了豌豆-小麦轮作对土壤有机碳、氮积累的促进作用，但在红豆草-小麦轮作中，红豆草对有限土壤水分的消耗促进了土壤有机碳、氮的积累。
(4)本区土壤C/N比一般稳定在8.3左右，作物和施肥措施对此影响不大。
(5)不同土地利用方式显著影响土壤微生物碳、氮含量。本区不同种植系统中土壤微生物碳含量占土壤有机碳含量的1-4％；微生物氮含量占有机氮含量的2-6％，微生物碳(氮)含量与有机碳(氮)含量具有显著的相关关系。相同管理条件下，微生物碳含量、微生物碳与有机碳比率变化方向与有机碳一致。但与土壤有机碳相比，微生物碳含量、微生物碳与有机碳比率变化幅度更大。因此可以利用土壤微生物碳含量、微生物碳与有机碳比率变化指示土壤质量变化。
(6)土壤水分含量依赖于年际、季节间降水量的变化。作物、施肥不改变土壤水分的变化模式，但影响土壤水分变化强度。休闲处理土壤水分含量季节性变化相对稳定。不同作物对土壤水分含量的影响强度顺序为：苜蓿＞小麦＞休闲地。不同施肥条件下，小麦地收获期土壤水分含量和播种前降水入渗深度大小为：不施肥处理、M、PM、N处理＞NP、NM、NPM处理。常年生长苜蓿的土壤，其水分含量显著低于休闲处理，降水入渗深度显著降低。苜蓿连作土壤中，施肥措施对土壤水分影响不大。
作物影响土壤NO_3-N的含量、迁移和积累。小麦连作中，施氮肥处理土壤NO_3-N含量显著升高，N处理的NO_3-N向下迁移深度显著大于NPM、MN处理。不同处理NO_3-N迁移深度与相应处理降水入渗深度基本一致。NO_3-N在土壤剖面(0-300cm)积累量可占到氮肥施用总量的12-55％。其NO_3-N积累量的大小顺序为：N处理＞NPM，MN＞NP，与相应
11 黄土旱源农田土壤有机碳、氮的演变与模拟
处理氮肥的利用率大小颀序相反。首捂连作中，NO3N迁移深度与降水的入渗深度一样显著
变浅，即使在NPM施肥条件下也没有发生NO3N在剖面中的显著积累。
（7）不同作物与施肥条件下，建立的土壤有机碳周转模拟模型（SCNC）与测定值相对
误差在t 10％范围内的模拟值可达到80％以上，而RohC－－26．3模型的模拟值与测定值相对误差
均在30％以上。
（8）目前水肥条件下，本区小麦产量远未达到水肥资源生产潜力。分析表明，休闲期门一9
月）降水量是影响产量的重要因素，但小麦产量只与7月降水量关系密切，因而提高8月、9
月降水的入渗、保持和利用是本区冬小麦产量进一步提高的重要途径。增施化肥是提高黄土
旱源区作物产量的主要措施。在目前氮肥投入水平（12 kgNha’）基础上，进一步增施氮肥，
小麦产量还具有一定的增产空间。
上述研究结果为黄士旱源区土壤生产力提高、土壤有机碳、氮的调控管理提供了新的理
论依据。
Soil productivity is strongly linked to soil organic matter (SOM) because SOM plays important roles in improving soil structure, soil water holding, infiltrating and capacities, and the nutrients supply to crops. It is significant that effect of land-use and management on soil organic matter turnover and land productivity is investigated for the sustainable agriculture and environment protection in gully area of the Loess Plateau. On the basis of long-term experiment, changes in soil organic C, N and dynamics of soil NOs-N of cropping systems were investigated in gully area of the Loess Plateau by means of analysis of the 15-year soil samples, crop yield hi following crop systems: 1) continuous winter wheat with unmanured (CK), nitrogen, organic manure (M), phosphorus and organic manure (PM), nitrogen and phosphorus (NP), nitrogen, phosphorus and organic manure (NPM), 2) continuous alfalfa, 3) pea-winter wheat rotation, winter wheat-sainfoin (Onobrychis viciifolid) rotation, and 4) bare fallow. A model was developed according to the organic C dynamics, and was tested by the long-term experiment. The potential yield of winter wheat and the approach were studied under the condition of local precipitation and fertilization. Some results were summarized as follows:
(1) Land-use and management affected soil organic C and N. Soil organic C and N content in the continuous cereal was constant at the level of 6.50 g kg"1 and 0.82 g kg"1, respectively. Cereal-pea rotation could contribute a little to the accumulation of soil organic C and N. Winter wheat-sainfoin (Onobrychis viciifolid) rotation or continuous alfalfa (Medicago sativd) could significantly increase the soil organic C, N. Soil organic C and N slowly decreased in fallow.
(2) Fertilization could evidently increase soil organic C, N. The increase of organic C varied from 9% to 13%, the increase of organic N varied from 22% to 35% among the different cropping systems.
(3) Limited rainfall affected the accumulation of soil organic C, N in the pea-winter wheat rotation. However, low soil moisture due to crop strong evapotranspiration contributed more to the accumulation of soil organic C, N in the winter wheat-sainfoin (Onobrychis viciifolid) rotation.
(4) Soil C/N ratio was constant at 8.3 in gully area of Loess Plateau, cropping and fertilization had little effect on the C/N ratio.
(5) Land-use and management could result in significant changes in soil microbial biomass C, N. Soil microbial biomass C accounts for 1-4% of soil organic C, soil microbial biomass N accounts for 2-6% of soil organic N among cropping systems. There was a significant correlation between soil microbial C (N) and organic C (N). The changes in microbial C, microbial C/organic C ratio, in response to changes in soil management, occurred in the same direction as did in the changes in soil organic C, but microbial C and microbial C/organic C ratio were sensitive to similar soil management. So the microbial C and microbial C/organic C ratio could serve as a sensitive indicator of changes in SOM.
(6) Soil moisture content depends on highly variable inter-year and inter-month rainfall. Cropping and fertilization could not change pattern of soil moisture change, and significantly affect intensity of soil moisture change. Effect of crops on soil moisture was in the order: continuous alfalfa > continuous winter wheat > bare fallow. Soil moisture on harvesting date and rainfall infiltration depth before winter wheat planting in continuous winter wheat was hi the order : unmanured, M, PM, N > NP, NM, NPM. Soil moisture content in continuous alfalfa was considerably lower than bare fallow, the rainfall infiltration depth in continuous alfalfa was reduced to 120 cm. Fertilization could not influence soil moisture in continuous alfalfa.
Soil managements (cropping, fertilization, etc.) had a little effect on NH4-N change. Cropping and fertilization had a significant effect on soil NOa-N transport and accumulation. NOa-N level was low and constant in fallow treatment.
(1)在黄土旱塬区，禾本科连作能够保持土壤有机碳、氮的含量(6.50gkg~(-1)、0.82gkg~(-1)；禾本科与豆科作物轮作土壤有机碳、氮积累略有提高；小麦与红豆草轮作或苜蓿连作对提高土壤有机碳、氮积累具有显著的促进作用；长期休闲土壤有机碳、氮呈缓慢降低趋势。
(2)不同作物系统中，增施化肥显著提高了土壤有机碳、氮的积累，有机碳、氮增幅分别为19-33％、22-35％。
(3)本区降水量的不足与多变限制了豌豆-小麦轮作对土壤有机碳、氮积累的促进作用，但在红豆草-小麦轮作中，红豆草对有限土壤水分的消耗促进了土壤有机碳、氮的积累。
(4)本区土壤C/N比一般稳定在8.3左右，作物和施肥措施对此影响不大。
(5)不同土地利用方式显著影响土壤微生物碳、氮含量。本区不同种植系统中土壤微生物碳含量占土壤有机碳含量的1-4％；微生物氮含量占有机氮含量的2-6％，微生物碳(氮)含量与有机碳(氮)含量具有显著的相关关系。相同管理条件下，微生物碳含量、微生物碳与有机碳比率变化方向与有机碳一致。但与土壤有机碳相比，微生物碳含量、微生物碳与有机碳比率变化幅度更大。因此可以利用土壤微生物碳含量、微生物碳与有机碳比率变化指示土壤质量变化。
(6)土壤水分含量依赖于年际、季节间降水量的变化。作物、施肥不改变土壤水分的变化模式，但影响土壤水分变化强度。休闲处理土壤水分含量季节性变化相对稳定。不同作物对土壤水分含量的影响强度顺序为：苜蓿＞小麦＞休闲地。不同施肥条件下，小麦地收获期土壤水分含量和播种前降水入渗深度大小为：不施肥处理、M、PM、N处理＞NP、NM、NPM处理。常年生长苜蓿的土壤，其水分含量显著低于休闲处理，降水入渗深度显著降低。苜蓿连作土壤中，施肥措施对土壤水分影响不大。
作物影响土壤NO_3-N的含量、迁移和积累。小麦连作中，施氮肥处理土壤NO_3-N含量显著升高，N处理的NO_3-N向下迁移深度显著大于NPM、MN处理。不同处理NO_3-N迁移深度与相应处理降水入渗深度基本一致。NO_3-N在土壤剖面(0-300cm)积累量可占到氮肥施用总量的12-55％。其NO_3-N积累量的大小顺序为：N处理＞NPM，MN＞NP，与相应
11 黄土旱源农田土壤有机碳、氮的演变与模拟
处理氮肥的利用率大小颀序相反。首捂连作中，NO3N迁移深度与降水的入渗深度一样显著
变浅，即使在NPM施肥条件下也没有发生NO3N在剖面中的显著积累。
（7）不同作物与施肥条件下，建立的土壤有机碳周转模拟模型（SCNC）与测定值相对
误差在t 10％范围内的模拟值可达到80％以上，而RohC－－26．3模型的模拟值与测定值相对误差
均在30％以上。
（8）目前水肥条件下，本区小麦产量远未达到水肥资源生产潜力。分析表明，休闲期门一9
月）降水量是影响产量的重要因素，但小麦产量只与7月降水量关系密切，因而提高8月、9
月降水的入渗、保持和利用是本区冬小麦产量进一步提高的重要途径。增施化肥是提高黄土
旱源区作物产量的主要措施。在目前氮肥投入水平（12 kgNha’）基础上，进一步增施氮肥，
小麦产量还具有一定的增产空间。
上述研究结果为黄士旱源区土壤生产力提高、土壤有机碳、氮的调控管理提供了新的理
论依据。
Soil productivity is strongly linked to soil organic matter (SOM) because SOM plays important roles in improving soil structure, soil water holding, infiltrating and capacities, and the nutrients supply to crops. It is significant that effect of land-use and management on soil organic matter turnover and land productivity is investigated for the sustainable agriculture and environment protection in gully area of the Loess Plateau. On the basis of long-term experiment, changes in soil organic C, N and dynamics of soil NOs-N of cropping systems were investigated in gully area of the Loess Plateau by means of analysis of the 15-year soil samples, crop yield hi following crop systems: 1) continuous winter wheat with unmanured (CK), nitrogen, organic manure (M), phosphorus and organic manure (PM), nitrogen and phosphorus (NP), nitrogen, phosphorus and organic manure (NPM), 2) continuous alfalfa, 3) pea-winter wheat rotation, winter wheat-sainfoin (Onobrychis viciifolid) rotation, and 4) bare fallow. A model was developed according to the organic C dynamics, and was tested by the long-term experiment. The potential yield of winter wheat and the approach were studied under the condition of local precipitation and fertilization. Some results were summarized as follows:
(1) Land-use and management affected soil organic C and N. Soil organic C and N content in the continuous cereal was constant at the level of 6.50 g kg"1 and 0.82 g kg"1, respectively. Cereal-pea rotation could contribute a little to the accumulation of soil organic C and N. Winter wheat-sainfoin (Onobrychis viciifolid) rotation or continuous alfalfa (Medicago sativd) could significantly increase the soil organic C, N. Soil organic C and N slowly decreased in fallow.
(2) Fertilization could evidently increase soil organic C, N. The increase of organic C varied from 9% to 13%, the increase of organic N varied from 22% to 35% among the different cropping systems.
(3) Limited rainfall affected the accumulation of soil organic C, N in the pea-winter wheat rotation. However, low soil moisture due to crop strong evapotranspiration contributed more to the accumulation of soil organic C, N in the winter wheat-sainfoin (Onobrychis viciifolid) rotation.
(4) Soil C/N ratio was constant at 8.3 in gully area of Loess Plateau, cropping and fertilization had little effect on the C/N ratio.
(5) Land-use and management could result in significant changes in soil microbial biomass C, N. Soil microbial biomass C accounts for 1-4% of soil organic C, soil microbial biomass N accounts for 2-6% of soil organic N among cropping systems. There was a significant correlation between soil microbial C (N) and organic C (N). The changes in microbial C, microbial C/organic C ratio, in response to changes in soil management, occurred in the same direction as did in the changes in soil organic C, but microbial C and microbial C/organic C ratio were sensitive to similar soil management. So the microbial C and microbial C/organic C ratio could serve as a sensitive indicator of changes in SOM.
(6) Soil moisture content depends on highly variable inter-year and inter-month rainfall. Cropping and fertilization could not change pattern of soil moisture change, and significantly affect intensity of soil moisture change. Effect of crops on soil moisture was in the order: continuous alfalfa > continuous winter wheat > bare fallow. Soil moisture on harvesting date and rainfall infiltration depth before winter wheat planting in continuous winter wheat was hi the order : unmanured, M, PM, N > NP, NM, NPM. Soil moisture content in continuous alfalfa was considerably lower than bare fallow, the rainfall infiltration depth in continuous alfalfa was reduced to 120 cm. Fertilization could not influence soil moisture in continuous alfalfa.
Soil managements (cropping, fertilization, etc.) had a little effect on NH4-N change. Cropping and fertilization had a significant effect on soil NOa-N transport and accumulation. NOa-N level was low and constant in fallow treatment.
引文
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