Comparison of different straw cycling modes in farmland ecosystems in Chengdu Plain based on emergy theory

The flow and transformation of energy and circulation of materials can reflect the basic functional processes of agricultural ecosystems. Energy analysis of agricultural ecosystems focuses on the characteristics of energy flow, which converts different resources into standard emergy. Thus energy conversion in intra-ecosystems or inter-systems can be explained and the optimized modes or structures can be chosen. This paper aimed at finding out the most applicable straw utilization mode for local agricultural ecosystems in Chengdu Plain. Chengdu Plain is one of the biggest grain and oil production bases with high straw quantity in China. How to make better use of the straw resources rather than excessive consumption of agro-ecological resources has attracted considerable interest in recent years. In this paper, the theory and method of emergy analysis were used to study different straw cycling modes in "rice-wheat rotation" farmland ecosystems in Chengdu Plain. Through the systematic positioning test and random sampling, emergy inputs and outputs of farm land production modes with different straw cycling ways in Chengdu Plain were investigated. The modes included regular fertilization mode (CF), straw returning mode (CFS) and mushroom residues utilization modes (FRs, in which three modes with different inputs were involved, FR₁, FR₂, FR₃). Results indicated that non-renewable emergy inputs of CF and CFS modes accounted for a large portion in all emergy input. This mainly depended on auxiliary emergy of non-renewable industry, which easily resulted into non-point source pollution. The sequence of total emergy values of different modes was CF > FR₂ > CFS > FR₁ > FR₃. Compared with CF and CFS modes, emergy input-output ratios of FR₁, FR₂ and FR₃ modes were relatively higher. Total emergy output and emergy input-output ratio of FR₂ mode were highest, reaching 2.20×10¹⁶ sej and 2.35, respectively. Mushroom residues utilization modes had high net emergy yield ratio with emergy self-sufficiency ratio above 50%. Emergy loading ratios of CF and CFS modes were relatively high, reaching 1.66 and 1.75, respectively. Emergy loading ratios of all the other modes of mushroom residues utilization were below 1. The sustainable indexes of CF, CFS, FR₁, FR₂ and FR₃ modes were 1.68, 1.42, 6.56, 5.75 and 4.07, respectively. While the CFS mode increased organic emergy input, it decreased net emergy yield ratio. The CFS mode not only had high emergy input and emergy loading ratio, but also under considerable environmental pressure. However, the FRs modes had less chemical fertilizer emergy input, low emergy loading ratio and high net emergy yield ratio. Due to the sustainable features and development potential, the FRs modes were recommended as applicable modes in Chengdu Plain. The FR₂ mode was considered as the most suitable straw utilization production mode.