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.