Stabilization of microbial residues in soil organic matter after two years of decomposition
Introduction
Soil organic matter (SOM) is not only an important carbon (C) pool in the global C cycle, but also a reservoir of nutrients, such as nitrogen (N) and phosphorus (P), for plant growth and soil microbial and animal activities (Elser et al., 2007; LeBauer and Treseder, 2008). Classical views and models assume that SOM mainly is derived from recalcitrant plant residues, while soil microorganisms are considered as the primary agents of SOM decomposition (Kögel-Knabner, 2002; Schmidt et al., 2011; Lehmann and Kleber, 2015). Yet, recent results show that microbial matter including their metabolic excretion and their senesced biomass (necromass) may be a significant part of SOM itself (Simpson et al., 2007; Miltner et al., 2012; Throckmorton et al., 2012; Schurig et al., 2013; Kögel-Knabner, 2017; Liang et al., 2017; Kästner and Miltner, 2018). Therefore, there is an emergent demand for a better understanding of SOM due to paradigm shift for its formation and stabilization (Schmidt et al., 2011; Lehmann and Kleber, 2015).
The structure and chemical composition of plant litter are different from that of soil microbes, which could result in different stability of SOM (Kögel-Knabner, 2002; Liang et al., 2017). For example, the global average C/N ratio of plant litter (~53) (Yuan and Chen, 2009) is much higher than that of microbial biomass (~7) (Xu et al., 2013). First, the decoupling of C and N cycling may happen if microbes select more N-containing materials for their demands. Then, once the selected plant litter is processed by microbes for their growth and the formation of microbial biomass, the molecular structure and characteristics of microbially-derived SOM may also be different from those of plant-derived SOM (Kögel-Knabner, 2002; Liang et al., 2017). Ultimately, whether microbial necromass could contribute significantly to SOM formation over a long-term period depends on its pool size and its decomposition rate in situ, which could be affected by climatic factors, the mineralogy of the soil, as well as the ‘sorptive affinity’ of a particular necromass materials to the solid phase (Castellano et al., 2015; Sokol et al., 2019). Although the active microbial biomass carbon is measured to be less than 2% (Dalal, 1998), the rapid turnover of this biomass could leave behind a large amount of necromass, which could contribute to more than 50% of SOM (Simpson et al., 2007; Liang et al., 2011). Once the microbial necromass is physically protected by soil mineral particles and aggregates, its mean residence time (MRT) in soil may be much longer than previously thought and necromass could be a major contribution to stable SOM (Simpson et al., 2007; Kästner and Miltner, 2018). Experiments on the decomposition of microbial necromass considering the actual conditions in soil including physical protection and the variation of climatic factors are needed to better understand SOM formation and to incorporate microbially-derived SOM into biogeochemical models.
Due to the individual composition of compound-classes of different decomposability, necromass of different microbial groups might reflect a wide range of decomposition rates (Kögel-Knabner, 2002; Six et al., 2006). Chitin is the basic unit of cell walls of fungi (Bartnicki-Garcia, 1968; Kögel-Knabner, 2002). Additionally, cell walls of some fungi also contain relatively high proportions of proteins and melanin (Kögel-Knabner, 2002). Some studies have suggested that melanin is a recalcitrant polymer in fungal necromass and may decompose slowly in soils (Fernandez and Koide, 2013; Fernandez et al., 2019). Bacterial cell walls are mainly composed of carbohydrate, which is built largely from amino sugars (Kögel-Knabner, 2002). Despite the different decomposability of microbial cell wall components, Throckmorton et al. (2012) found no difference in the MRT of necromass C in soil among bacteria, actinobacteria and fungi. If there is a difference in the MRT of necromass N among diverse microbial groups is still unclear.
Although previous researchers have conducted pioneering works to estimate microbial necromass C decomposition rate using 13C and 14C isotope labeling techniques, and NMR spectroscopy in laboratory (Nelson et al., 1979; Jawson et al., 1989; Kindler et al., 2006; Miltner et al., 2009; Spence et al., 2011; Throckmorton et al., 2012), the in situ decomposition rate of microbial necromass N has never been studied. Previous studies have shown that the portion of microbially-derived soil organic N (SON) to total SON may be higher than the portion of microbially-derived SOC to total SOC (Simpson et al., 2007), pointing to the importance of microbially-derived N to soil N cycling. Although C and N are often coupled in different types of compounds of microbial detritus, selective preservation of either C or N during the decomposition of these necromass materials may decouple C and N decomposition, resulting in different decomposition rate of microbial C and N in SOM (Knowles et al., 2010; Veuger et al., 2012). However, at present, a quantitative assessment of the decomposition rate of microbial necromass N is still lacking. In addition, previous studies on microbial necromass C have mostly been done in the lab and at short time scales (Kindler et al., 2006; Schweigert et al., 2015), which may not represent the real conditions in the field.
Here we utilized a15N labeling approach to explore the in situ turnover of bacterial, fungal, and actinobacterial necromass N in a temperate forest soil. Four members of each microbial group were isolated from the soil, labeled with 15N, sterilized and incubated in a temperate Korean pine and broad-leaved mixed forest. We traced the labeled 15N in microbial necromass into bulk total soil N (TN), soil microbial biomass N (MBN), soil inorganic N (NO3−-N and NH4+-N), and gaseous N (N2O) pools for 803 days after the addition of tracer. A decay model was used to estimate MRT of microbial necromass 15N in soil to compare it with the MRT of necromass 13C from previous studies on 13C-labeled necromass (Throckmorton et al., 2012). Additionally, the necromass N turnover rate was estimated by introducing an analytical model that included necromass N turnover, necromass N production, biomass N turnover and microbial NH4+ immobilization processes. We further compared the estimated necromass N decomposition rate with the decomposition rate of plant litter N in the same site. We hypothesized that: (1) the MRT of necromass N in soil would be longer than 2 years (the duration of a necromass C decomposition in the filed (Throckmorton et al., 2012) and necromass N is an important contributor to soil organic N; and (2) necromass N of three groups of microbes (bacteria, fungi, actinobacteria) would differ in their decomposition rate in soil due to their different chemical materials of cell walls.
Section snippets
Study site and experimental design
The study was conducted at the Changbai Mountain Forest Ecosystem Research Station, which is located in the east of Jilin Province in northern China (42.70° N,127.63° E). This region is characterized by a typical temperate climate, with annual mean precipitation and temperature at 745 mm and 3.6 °C, respectively. The growing season is from May to September, with a mean temperature of 16.7 °C. The study site is in a natural Korean pine and broad-leaved mixed forest at 700–720 m above sea level,
Initial necromass characteristics
Four isolates within each microbial group were selected from more than one hundred cultured isolates (Table 1). The average initial N content of fungal necromass was 5.3%, which was lower than that of bacterial (9.2%) and actinobacterial (10.3%) necromass. Additionally, the initial C/N ratio of fungal necromass was slightly higher than that of bacterial and actinobacterial groups. The atom % 15N was similar among the three groups, with an average atom% 15N at 65.0 ± 0.9%.
Decomposition of necromass 15N and litter 15N
The labeled necromass N
Discussion
The analytical model results suggest that the overall decomposition rate of necromass N ranged from 3.62 to 4.08 yr−1 in our study site, with an average value of 3.84 yr−1. The two-pool parallel model showed significantly different decomposition rate constants between the labile pool and the resistant pool (Table 2). While the labile pool had a very fast decomposition rate, the resistant pool turned to be relatively slowly decomposed. Nevertheless, 51.1%–56.3% of necromass 15N was recovered in
Acknowledgements
We appreciate the many helpful comments from two anonymous reviewers that greatly improved the manuscript. We thank Sun, Hao and Li, Xu for the assistance on isolation and enrichment of soil microbial strains, Ci Sun, Zhenzhen Fan, Ying Tu, Linlin Song, Ziping Liu and Qu, Lingrui for laboratory analyses, Xianlei Fan for model running and Dr. Ember Morrissey for the comments on the preliminary version of the manuscript. This work was financially supported by the Key Research Program of Frontier
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