Dead microbes [microbial necromass or microbe corpses] are a large and significant component of soil organic carbon [SOC], and thus are crucial for long-term carbon [C] sequestration and stabilization. It is reasonable to assume that cropland management practices will affect the accumulation of this material and its subsequent contribution to SOC. Just what practices have this effect and the amount of that effect are important questions to consider when attempting to increase SOC on sites where crops are grown.
Information contained in an article titled “Microbial necromass in cropland soils: a global meta-analysis of management effects” by Zhou et al. provides some answers to these questions. The article was published in the journal Global Change Biology in Feb. 2023. Major points from that article follow.
• Results in 61 peer-reviewed journal articles that were published prior to Aug. 2022 were selected for this meta-analysis. A total of 481 paired observations that reported the effects of management practices on microbial necromass accumulation were selected from these articles.
• Most of the studies were conducted in East Asia, North America, and Western Europe.
• The focus of the analysis was to determine the impact of cropland management practices that included nitrogen [N] fertilization, application of manure from agricultural enterprises, straw [residues of myriad plant species] and biochar soil amendments, no or reduced tillage [NT/RT], and cover crops on soil microbial necromass accumulation.
• All investigated management practices except biochar addition increased total microbial necromass C in cropland soils by 12-21%. The order of increase was 1) straw and manure additions–21%, 2) NT/RT–20%, 3) cover crops–14%, and 4) N fertiliztion–12%.
• Bacterial necromass C was greatest with manure application, whereas fungal necromass C was greatest with straw application.
• Only straw addition and NT/RT increased the total microbial necromass C contribution to SOC.
• Responses of microbial necromass C to management depended on climate [temperature, rainfall, humidity], soil properties [texture, initial SOC content, pH], quality and quantity of C input, and experiment duration [short term (< 3 years) vs. long term (> 10 years)].
• The analyzed data from the selected articles validated that cropland management practices affected soil microbial biomass, and thus subsequently influenced the production of microbial necromass. There was a direct positive effect of microbial biomass on necromass accumulation.
• NT/RT was the most efficient practice for the accumulation of microbial necromass, followed by cover crops and straw addition. N fertilization had the smallest accumulation efficiency. The accumulation efficiency of fungal necromass was about 6.4 times greater than that of bacterial necromass under NT/RT, followed by cover crops.
• Overall, this meta-analysis indicated the following. 1) N fertilization and straw addition increased microbial necromass accumulation in semi-arid and cool climates, whereas NT/RT and cover crops were more effective at doing this under humid vs. semi-arid conditions [an important finding for the humid midsouthern U.S.]. 2) Microbial necromass accumulation was closely connected with the amount of living microbes in the soil and with SOC content. Thus, conservation management practices applied to cropland soils increase microbial biomass, which in turn enhances necromass formation and accumulation, thereby supporting the buildup of SOC.
• In conclusion, all commonly used conservation management practices except biochar addition increased microbial necromass accumulation in cropland soils. However, the quantity of the increase was dependent on climate and edaphic conditions at the cropping site.
When soil health is discussed, it invariably leads to/should lead to determining ways to enhance the soil microbial population that is instrumental in maintaining healthy and productive soils. The above information lends further credence to the importance of 1) the soil microbial population for enhancing C sequestration in soil, and 2) the need to apply conservation agricultural practices over a long period (~> 10 years) in order to realize their full potential for improving the environment of soil that is used to grow crops.
Takeaway. Cropland management practices identified as contributors to an increased soil microbial population and its subsequent necromass should be adopted and used on a global scale. Click here and here to access White Papers on this website that provide details about soil health and the soil biota.
Composed by Larry G. Heatherly, Mar. 2023, firstname.lastname@example.org