Microbial community structure in rhizosphere soils of long-term continuously cropped processing tomato based on PLFA method

Xinjiang Uygur Autonomous Region is a major production base of processing tomato in China. In the effort to meet market demand for processing tomato, mono-cropping has been widely adopted. Unfortunately, this phenomenon has become the dominant factor limiting the stable production and yield of tomato in the region. Here, we conducted a field study to determine the impact of continuous cropping over the long-term on microbe community structure in rhizosphere soil of processing tomato using phospholipid fatty acid (PLFA) and chloroform fumigation extraction method. The mono-cropping field experiment started in 2007 at a station belonging to the College of Agriculture of Shihezi University. The processing tomato cultivar used in the experiment was 'Ligeer 87-5'. Soil samples were collected for analysis from plots with different cultivation histories (3, 5 and 7 years of continuous cropping) and a control plot that was under fallow for 3 years. The results showed that soil microbial biomass C (SMBC), soil microbial biomass N (SMBN) and soil microbial biomass entropy (qMB) significantly decreased, while soil microbial biomass C/N increased with increasing duration of continuous cropping (P < 0.05). However, soil microbial biomass P (SMBP) exhibited a different response to both continuous cropping and various growth stages. PLFA analysis indicated that continuous cropping significantly increased fungal PLFAs, whereas the reverse trend was observed for bacterial PLFAs, total PLFAs and the ratio of bacterial PLFAs to fungal PLFAs. However, actinomycetous PLFAs had no regular change with increasing duration of continuous cropping. After 7 years of continuous cropping, bacterial PLFAs and total PLFAs amount decreased by 62.9% and 50.3% (P < 0.05), respectively, but fungal PLFAs amount significantly increased by 60.2% compared with control. Based on diversity index analysis, Shannon-Wiener index, Simpson index, Brillouin index and Pielou index all initially increased and then decreased with increasing years of continuous cropping of processing tomato. Soil microbial diversity index was highest in the treatment of 3 years continuous cropping and was lowest for the treatment of 7 years continuous cropping. It was concluded that microbial community diversity and uniformity decreased with increasing of continuous cropping years in this area. Correlation analysis showed significant correlation among PLFAs of bacteria, fungi and actinomycetes, total PLFAs, soil microbial biomass and soil fertility, which indicated that soil microbial biomass was highly related with soil fertility. Therefore, soil microbial biomass could be used as an available biological index for the evaluation of soil fertility. The results suggested that years of mono-cropping had a major influence on microbial community structure and soil microbial biomass in rhizosphere soil of processing tomato, which in turn limited sustainable development of processing tomato.