京郊典型集约化"农田-畜牧"生产系统氮素流动特征

城郊畜牧业的集约化发展在满足人们日益增长的肉蛋奶等动物性产品需求的同时,也带来了巨大的环境压力。本文利用养分流动和模型分析的方法,分析北京市郊区某村三种不同类型"农田-畜牧"生产系统(大型集约化种猪场、种养结合小规模生态养殖园和集约化单一种植区)的氮素流动特征。结果表明:饲料是集约化种猪场和生态养殖园氮素输入的主要来源,饲料投入氮量分别为12469.0和9268.5 kg.hm-.2a-1);集约化种猪场农牧体系间生产脱节,致使农田氮素输入主要依赖于化肥投入,化肥氮输入量(435.0 kg.hm-.2a-1)占农田氮素输入量的82.7%。相反,生态养殖园农牧体系结合紧密,猪粪尿氮还田比例达28.6%,这使得园区化肥氮输入量仅为135.0 kg.hm-.2a-1,因此畜禽粪尿的合理循环利用可作为减少化肥投入的有效途径;集约化种猪场、生态养殖园和单一种植区农牧生产系统氮素利用效率分别为18.8%、20.6%和17.3%,均处于较低水平;集约化种猪场猪粪尿在猪舍和储藏处理过程的氮素损失率为15.8%和25.4%,分别低于小规模生态养殖园相应损失率约8.7和4.8个百分点;生态养殖园粪肥氮还田量高,导致农田氮素盈余量高达1962.8 kg.hm-.2a-1,未能实现生态型养殖的理想效果,优化氮素管理、确定合理的消纳畜禽粪尿的农田面积和调整畜禽养殖密度是解决该问题的关键。

Nitrogen flows in intensive "crop-livestock" production systems typically for the peri-urban area of Beijing

A further increasing demand for animal products is anticipated owing to the growing population, rapid urbanization and improvement of living standards in China. This is leading to the development of large-scale livestock husbandry in peri-urban areas at an unprecedented rate. However, the rapid development of intensive animal production has resulted in greatly negative effects on the environment. Linking cropping and animal production systems is very important to realize the improvement of nutrient use efficiency and reduction of resource use. Consequently, a systematic research on "crop-livestock" production systems is urgently required to provide a scientific basis for reducing nutrient losses in China. In this study, three types of "crop-livestock" systems in a village typical for the peri-urban area of Beijing were selected. The first type was an "intensive pig farm", which included pig breeding and crop production; the second type was so-called "ecological feeding gardens" consisting of 160 small pig holders; the third one was a "single cropping region" which had no animal production. The characteristics of nitrogen (N) flows in these three systems were analyzed using the nutrient flows method. Of the input components, purchased feeds (IN1), chemical fertilizers (IN2), purchased animal manure (IN3), atmospheric deposition (IN4), irrigation water (IN5) and asymbiotic nitrogen fixation (IN6) were considered, while sold animal products (OUT1), crop products (OUT2) including economic products (OUT2a) and byproducts (OUT2b), animal manure (OUT3) and nitrogen losses from animal excreta during animal housing (OUT4) and manure storage (OUT5) were defined as output components of the "crop-livestock" production system. In addition, self-produced feeds (Inter1), recycled manure (Inter2) and crop residues incorporated into fields (Inter3) were considered as the internal components. Nitrogen use efficiencies ((OUT1+OUT2a)/total N inputs (IN1-IN6)) were used to estimate the sustainability of the "crop-livestock" systems. The results showed that purchased feeds (IN1) were the dominant component of N inputs to the "crop-livestock" systems both in "intensive pig farm" and "ecological feeding gardens" types. Nitrogen input in purchased feed was 12469.0 kg·hm^-2·a^-1, accounting for 95.9% of total N input in the former type, and in the latter type it was 9268.5 kg·hm^-2·a^-1, which accounted for 97.6% of total N input. The N application rate of chemical fertilizers (IN2) in the "intensive pig farm" was 435.0 kg·hm^-2·a^-1, comparing to the corresponding value of 135.0 kg·hm^-2·a^-1 in the "ecological feeding gardens". The lower chemical fertilizer N application rate in the latter was attributed to the recycling of manure N (Inter2), where 28.6% of the total excreted N was applied to crops. Consequently, this confirmed that the recycling of manure N is an efficient way to reduce the application rate of chemical N fertilizer. However, low nitrogen use efficiencies of 18.8%, 20.6% and 17.3% for the "intensive pig farm", "ecological feeding gardens" and "single cropping region" types, respectively, were found over the study period. This can be partly explained by the relatively high N losses from animal excreta during animal housing (OUT4) and manure storage (OUT5), accounting for 15.8% and 25.4% of animal excreted N in the "intensive pig farm", respectively, even though they were 8.7 and 4.8 percentage points lower than those from the "ecological feeding gardens". Positive cropland N balances resulted across all three types of "crop-livestock" systems, especially in the "ecological feeding gardens" where the annual nitrogen surplus reached 1962.8 kg·hm^-2·a^-1, indicating that the purpose of "ecological" feeding was not achieved as planned. It is concluded that a proper animal stocking density associated with the optimization of N management and an appropriate amount of cropland for animal manure recycling are the keys to solve this issue.