Microbial life-history strategies play a pivotal role in regulating soil nutrient cycling and plant productivity in agro-ecosystems [1]. Bacterial strategies can be inferred by their rRNA operon (rrn) copy number in the genome, which is related to protein synthesis and growth [2, 3]. Bacterial genomes usually contain multiple rrn copies (i.e. from 1 to 15)[4], for example, certain Steroidobacter and Rhodopirellula isolates contain one rrn copy, and some Bacillus and Paenibacillus isolates contain 10 copies per genome [5]. Bacteria with more rrn copy numbers (i.e. copiotrophs) grow quickly when resources are sufficient, while those with fewer rrn copy numbers (i.e. oligotrophs) exhibit higher carbon use efficiency [6, 7]. Growth and nutrient use strategies in bacteria depend on the pool, flux and availability of nutrients in the soil [8]. Thus, understanding the rrn responses to nutrient addition is indicative of microbial life-history strategies and regulated ecological functions such as nutrient cycling, carbon sequestration and plant growth promotion [9].
Agro-ecosystems are subjected to global intensive management practices such as long-term nitrogen (N) and phosphorous (P) fertilizations to increase crop yield. These approaches not only cause environmental pollution [10], but also alter soil CNP stoichiometry and other properties (e.g., soil pH), strongly affecting microbial communities. Increased rrn copy numbers have previously been reported in culture media, digesters, aquatic systems and also soils after short-term nutrient addition [11, 12, 13]. Long-term disturbance may lead to a contrasting change in microbial communities, while the responses of microbial life-history strategies to long-term fertilization and underling mechanisms remains unclear.
Unlike short-term management, the effects of a single nutrient deficiency, i.e. nutrient imbalance, on microbial life-history strategies might be magnified under long-term fertilization, as microbial requirements for C, N and P are usually restricted in a stable ratio of 60:7:1 [14, 15]. Without N or P additions, the organic C decomposition and soil microbial growth (especially copiotrophs) are reduced, even if C resources are in excess, and most organic C will be combusted during overflow respiration. When nutrient limitation is relieved, increased decomposition of soil organic matter by copiotrophs may release more available resources, accelerating their reproduction rates and their competitiveness for nutrients compared with other bacterial groups [1, 16].
Furthermore, long-term fertilization, especially with N input, always leads to severe soil acidification, decreasing the soil pH by > 0.5 units in most agricultural soils [17, 16, 18]. Given that bacterial diversity increases with soil pH [19, 20, 21, 22], soil acidification is an environmental stress that shifts microbial life-history strategies. Soil acidification may prefer the growth of pH-resistant bacteria such as Acidobacteria, whose abundances are not affected by soil pH or increase as soil pH elevates [23]. Some evidence supports that microorganisms containing more rrn copy numbers are likely more resistant to environmental stress [24]. Moreover, rare taxa in very low abundance (usually as oligotrophs containing low rrn copy numbers) may become extinct if their habitats deteriorate due to soil acidification [25, 26]. Thus, the altered microbial community caused by environmental stress may increase the community-level rrn copy number.
Here, we investigated the effects of long-term fertilization, i.e., N and P inputs over > 30 years, on the community-level life-history strategies of bacteria in soils within three agro-ecosystems that varied in soil-type and fertilization strategy. Our aims were to 1) investigate the responses of soil microbial life-history strategies to long-term N and P fertilizations and determine whether the responses were consistent across diverse agro-ecosystems; and 2) explain the underlying mechanisms associated with soil nutrient balance and soil acidification. We hypothesize that long-term N fertilization increases the community-level rrn copy number by adjusting nutrient imbalances, but causes soil acidification, favoring the growth of specific taxa with high rrn copy numbers. In contrast, the long-term P fertilization would affect microbial strategies mainly by adjusting nutrient imbalances. Understanding microbial strategies under long-term fertilization and the association with soil stoichiometry and environmental stress is pivotal to the management of soil productivity and ecosystem functioning.