Responses of absolute and specific enzyme activity to consecutive application of composted sewage sludge in a Fluventic Ustochrept

Composted sewage sludge (CS) is considered a rich source of soil nutrients and significantly affects the physical, chemical, and biological characteristics of soil, but its effect on specific enzyme activity in soil is disregarded. The present experiment examined the absolute and specific enzyme activity of the enzymes involved in carbon, nitrogen, and phosphorus cycles, the diversity of soil microbial functions, and soil community composition in a Fluventic Ustochrept under a maize—wheat rotation system in North China during 2012–2015. Application of CS led to increase in MBC and in its ratio to both total organic carbon (TOC) and microbial biomass nitrogen (MBN). Absolute enzyme activity, except that of phosphatase, increased in CS-treated soils, whereas specific activity of all the enzymes declined, especially at the highest dose of CS (45 t ha−1). The diversity of soil microbial community also increased in CS-treated soils, whereas its functional diversity declined at higher doses of CS owing to the lowered specific enzyme activity. These changes indicate that CS application induced the domination of microorganisms that are not metabolically active and those that use resources more efficiently, namely fungi. Redundancy analysis showed that fundamental alterations in soil enzyme activity depend on soil pH. Soil specific enzyme activity is affected more than absolute enzyme activity by changes in soil properties, especially soil microbial activity and composition of soil microflora (as judged by the following ratios: MBC/TOC, MBC/MBN, and TOC/LOC, that is labile organic carbon) through the Pearson Correlation Coefficient. Specific enzyme activity is thus a more accurate parameter than absolute enzyme activity for monitoring the effect of adding CS on the activities and structure of soil microbial community.

堆肥化污水污泥（Composted sewage sludge, CS）被视为富含土壤养分的资源，可显著影响土壤的物理、化学与生物学特性，但其对土壤特定酶活性的影响却常被忽视。本试验于2012-2015年在华北地区的玉米-小麦轮作系统下，针对潮干始成土（Fluventic Ustochrept），探究了参与碳、氮、磷循环的酶的绝对酶活性与比酶活性，以及土壤微生物功能多样性与土壤群落组成。施用CS可提升土壤微生物生物量碳（Microbial Biomass Carbon, MBC），及其与总有机碳（Total Organic Carbon, TOC）、微生物生物量氮（Microbial Biomass Nitrogen, MBN）的比值。在CS处理的土壤中，除磷酸酶外，所有酶的绝对酶活性均有所提升，但所有酶的比酶活性均出现下降，尤其在最高施用量（45 t·ha⁻¹）下降幅更为显著。CS处理土壤的微生物群落多样性亦有所提升，但当CS施用量较高时，由于比酶活性降低，其功能多样性出现下降。上述变化表明，施用CS会诱导代谢不活跃微生物及资源利用效率更高的真菌成为土壤微生物群落的优势类群。冗余分析结果显示，土壤酶活性的根本性变化取决于土壤pH值。通过皮尔逊相关系数分析可知，相较于绝对酶活性，土壤性质变化——尤其是土壤微生物活性及微生物区系组成，可通过MBC/TOC、MBC/MBN及TOC/易变性有机碳（Labile Organic Carbon, LOC）比值反映——对土壤比酶活性的影响更为显著。因此，相较于绝对酶活性，比酶活性是监测CS添加对土壤微生物群落活性与结构影响的更精准参数。