Root litter decomposition is a major component of carbon (C) cycling in
grasslands, where it provides energy and nutrients for soil microbes and
fauna. This is especially important in grasslands where fire is common and
removes aboveground litter accumulation. In this study, we investigated
whether fire affects root decomposition and C flow through the belowground
food web. In a greenhouse experiment, we applied
Soils contain an immense diversity of soil microorganisms and soil fauna,
and are of key importance to terrestrial ecosystems nutrient cycling and
carbon (C) storage (Wall et al., 2010; Wall, 2004; Bardgett, 2005; Smith et
al., 2015). Understanding the roles of the soil food web in regulating
belowground processes of decomposition, nutrient cycling, and C cycling is
recognized as a hot topic of research in soil ecology (Bardgett and Cook,
1998; Holtkamp et al., 2008, 2011; Carrillo et al., 2011; Osler
and Sommerkorn, 2007; Bardgett et al., 2013; van der Putten et al., 2013).
This is especially because we still lack a clear understanding of how soil
fauna contributes to these ecosystem processes and the ecosystem services
they provide (Nielsen et al., 2011; Carrillo et al., 2011; Brussaard,
1998; Bardgett and Cook, 1998; Smith et al., 2015). Within the soil fauna,
nematodes, which can occur at densities of approximately 1 million to
10 million m
In tallgrass prairie ecosystems, fire is a historical disturbance that has ecosystem level effects on plant dynamics and other processes (Knapp et al., 1998). Frequent fires can have large effects on plant productivity, plant community composition, and root properties (Kitchen et al., 2009; Knapp et al., 1998), which can significantly alter ecosystem processes such as litter decomposition and C cycling (Ojima et al., 1994; Johnson and Matchett, 2001; Soong and Cotrufo, 2015). Litter decomposition is an important component of belowground C cycling and root litter C provides a major energy source for soil biota (Eisenhauer and Reich, 2012). Since fire removes aboveground litter, and enhances root growth and belowground C allocation, root detrital input may be an even more important energy source for decomposer food webs in frequently burned grasslands (Seastedt et al., 1991; O'Lear et al., 1996). Furthermore, root decomposition studies have been highlighted as crucial because root litter is a major source of soil C (Rasse et al., 2005), contributing more than aboveground litter, and very little research has been done on the topic (Schimel and Schaeffer, 2012).
The belowground effects of fire have additional impacts on soil biodiversity and their functions. Burning causes changes in the soil surface energy budget by removing plant litter accumulation (O'Lear et al., 1996; Knapp and Seastedt, 1986). This leads to changes in soil conditions, such as nitrogen content, carbon content, temperature, and moisture, which could impact microbial and faunal activities or change detritivore community composition. Microbial community compositional changes have been reported as a result of fire: for example, fire alters microbial community composition by reducing Gram-negative and Gram-positive bacteria (Docherty et al., 2011) and increasing arbuscular mycorrhizae (Hamman et al., 2007). Also, fire initially impacts the overall abundance of nematodes negatively (Whitford et al., 2014), but this rebounds quickly and certain groups, such as colonizing bacterivore nematodes, respond positively after fire (Jones et al., 2006; Todd, 1996). Such changes in soil community composition have been shown to impact litter decomposition (Verhoef and Brussaard, 1990). While most litter decomposition is ultimately the product of soil fungal and bacterial metabolic activities, soil fauna also plays a role in litter decomposition by influencing these microbial activities and altering litter chemical composition (Coleman and Crossley, 1996; Verhoef and Brussaard, 1990; Petersen and Luxton, 1982; Xin et al., 2012; Mamilov, 2000; Coleman and Hendrix, 2000; Carrillo et al., 2011; Swift et al., 1979; Soong et al., 2016). However, little is known about how fire impacts both soil microbial and faunal community function or if frequently burned grasslands' soil communities are more specialized to decompose root litter than unburned soil communities.
Addition of
This project was designed to trace C from decomposing root litter into
components of the soil food web over time for annually burned (AB) and infrequently
burned (IB) prairie soils. Our conceptual approach included the production
of a
Soil samples were taken from historically unplowed tallgrass prairie at the
Konza Prairie Long Term Ecological Research (LTER) station in eastern
Kansas, USA (39
Soil cores (10 cm deep
Field temperature and moisture were measured at time of soil collection for
both AB and IB soils. Soil temperature was recorded in the field and daily
during the greenhouse incubation using a temperature probe coupled to a PP
system (PP Systems, SRC-1). Initial soil moisture was determined by
gravimetric water content (GWC) by subtracting the oven-dry weight of soil
(105
Prior to experiment setup,
Our experimental design consisted of two burn treatments and two litter
treatments in a fully factorial design (2 burn treatment
To assess decomposition and biotic community changes over time,
six destructive harvests occurred over 180 days, i.e., at 3, 10, 21, 35, 90, and
180 days. At each harvest date, four replicates of each of the four
treatments were harvested for analyses of soil, root litter, and biota.
Specifically, the litterbag was carefully removed from the soil and set
aside and then each soil core was removed from the collar, placed into a sterile
plastic bag, and well mixed to homogenize soil. Each homogenized soil sample
was subsampled for PLFA analysis and nematode extraction. The roots were
retrieved from the litterbag before drying in an oven at 45
Microbial community structure was assessed by phospholipid fatty acid (PLFA)
analysis. We ran three out of the four replicates (chosen at random) for
PLFA analysis due to the expense and time required to run these analyses.
Soil subsamples for PLFA analysis were sieved to 2mm, with any visibly
remaining plant material carefully removed with forceps. The PLFA
extraction, quantification, and
A number of PLFAs were selected as biomarkers for different microbial groups to investigate the soil microbial community composition (Frostegård and Bååth, 1996; Zelles, 1999). The PLFAs i15:0, a15:0, i16:0, a17:0, and i17:0 were selected to estimate the abundance of Gram-positive bacteria, and cy17:0, cis16:1n9, 18:1n11, and cy19:0 for Gram-negative bacteria. Fungal abundance was based on cis18:1n9 and cis18:2n9,12, and methylated PLFAs 10Me-16:0, 10Me-17:0, and 10Me-18:0 were used as indicators of actinobacteria.
The abundance of individual PLFAs was calculated (ng g
For both AB and IB treatments, soil nematodes were extracted from each soil sample by a modified Baermann funnel method in deionized water after Hooper (1970). A subsample of 100 g of soil was placed onto the Baermann funnels and an aliquot of water and nematodes removed daily for 3 days.
Nematodes were counted, identified, and sorted using an inverted microscope
(Olympus CKX41, 200
The absolute abundance of individual nematode groups was calculated (number nematodes per kilogram dry soil). Changes in the nematode community composition were evaluated based on relative nematode abundance data, which were calculated by dividing the absolute abundance of a nematode group by the sum of the absolute abundance of all nematode groups.
The isotope ratios are reported in terms of
The proportion of root litter carbon incorporated into nematode and
microbial tissue (
The amount of root-derived C incorporated into individual PLFAs and nematode
groups was calculated by multiplying the
A distance-based redundancy analysis (dbRDA) was used to evaluate differences in microbial and nematode community composition among fire and litter treatments. The dbRDA is a multivariate approach that is widely accepted and used for ecological studies to evaluate multispecies responses to several factors (Legendre and Anderson, 1999). For our dbRDAs, PLFA and nematode relative abundance data (mol % of each identified PLFA or nematode group) were used in two dbRDA models. A distance matrix was calculated for each community using the Bray–Curtis measure to model the species matrix. A principal coordinate analysis was performed on the distance matrix and the resulting eigenvalues were applied to a redundancy analysis. Ordination plots were drawn with ellipsoids (representing a 95 % confidence interval) around the multivariate community groups. The dbRDA and subsequent drawing of ordination plots were performed using R (R Core Team, Vienna, Austria).
Community structure plots depicted from results of the
distance-based redundancy analysis performed on relative abundance of PLFA
biomarkers
Burn treatment had a significant effect on the soil community. The dbRDA
revealed that AB and IB community compositions of microbes and nematodes
were significantly different (Fig. 1a and b, respectively). For
microbes, the differences in community composition were driven by biomarkers
for fungi (
Abundances of PLFA biomarkers for the annual burn
With the addition of root litter to the soil, microbial and nematode communities were changed (Fig. 1). The dbRDA revealed that the microbial community structure of AB changed significantly with the addition of root litter, while IB did not (Fig. 1a). Also, the AB and IB microbial communities became slightly more similar with root litter addition, yet these were still significantly different (Fig. 1a). As for abundance, 180 days after litter addition, there were no significant differences in abundance for any functional group for the IB or AB treatment relative to the control (Fig. 2).
Change in nematode trophic group abundance (nematodes/kg dry
soil) over time for both
Neither AB nor IB nematode community composition was significantly changed
with the addition of root litter, but there was a general shift in the
community (Fig. 1b) and total abundance of nematodes differed significantly
through time (Fig. 3). The shift in the litter addition communities was
largely driven by bacterivore nematodes (Fig. 1b), and the abundance of
bacterivore nematodes significantly increased with root litter addition for
both treatments (
Significantly more root litter mass was lost for the AB treatment than the
IB treatment (
Soil biota (both microbial PLFA biomarkers and nematodes) assimilated root
litter
The microbial biomarkers assimilation of root litter C increased significantly over time for both treatments (Fig. 4b). Despite higher total PLFA concentration in the infrequent burn treatment, the microbial pool of root litter C was not different between treatments. While there was generally more root-litter-derived C in the PLFAs initially (days 3, 10, 21) for IB and a lag in root litter C uptake for AB (Fig. 4b), the effect of burn treatment and the interaction of burn treatment and time was not significant for this pool of C. Also, the flow of C through the different groups of the microbial community was similar for each burn treatment (Fig. 5). In general, Gram-negative bacteria dominated the C uptake initially (days 3 to 21), and this shifted to Gram-positive dominance by 35 days for both burn treatments (Fig. 5). Fungal use of root litter C differed slightly for the burn treatments, with fungi from the AB treatment increasing in root litter C over time (Fig. 5c and d).
Root litter C dynamics during incubation for the annual burn and
infrequent burn treatments. Data are averages with standard error bars. The
root litter carbon
The nematodes' assimilation of root litter C also increased significantly over time for both treatments (Fig. 4c). While the burn treatment alone was not significant, the interaction of time and burn treatment was highly significant for the nematode C pool. At day 35 and 90, the nematode root-litter-derived C pool was significantly higher for AB than the IB treatment (Fig. 4c). The flow of C through the nematode community also differed somewhat (Fig. 5a and b). For both treatments, bacteria and, correspondingly, bacterivore nematodes played a dominant role in root litter C utilization for both AB and IB soils (Fig. 5). Bacterivore nematodes dominated the nematode community in abundance and incorporated the greatest amount of root litter C overall; however, the other trophic groups differed between burning treatment. For the IB treatment, omnivore and predator nematodes utilized a significant portion of root litter C by 35 days after litter addition, but not for AB. For the AB treatment, fungivore nematodes significantly incorporated root litter C from day 3, but not for the IB treatment.
When we looked at the proportions of root litter C incorporated into
individual group's biomass, there were differences between burn treatments.
Overall, fungivore nematodes, saprotrophic fungi (
Root litter C incorporation into microbial PLFAs and nematode
trophic groups. Panels
Burning has significant impacts on the belowground community, including soil microbes and soil nematodes. We found that both soil microbial and nematode community structure differed with long-term burn treatments (Fig. 1), with the AB treatment also showing reduced microbial biomass (via PLFA methods). These findings support our first hypothesis, that different burn treatments would house different soil communities, and confirmed previous observations. In particular, Todd (1996) showed that bacterivore nematodes respond positively to frequent fire, while predator nematodes do not. Jones et al. (2006) later corroborated that study via molecular methods. Additionally, fire has been shown to reduce overall microbial biomass and specifically affects Gram-negative and Gram-positive bacteria and fungi (Docherty et al., 2011; Ajwa et al., 1999). Such differences in the soil communities have implications for ecosystem function, such as impacts on organic matter decomposition (Verhoef and Brussaard, 1990).
Overall mean fraction (
Root litter mass loss was greater for the AB treatment, confirming our second hypothesis, i.e., that decomposition would be greater for the AB treatment. These results were in agreement with the observed higher aboveground litter respiration in the AB as compared to the IB site (Soong and Cotrufo, 2015). Yet, in a root decomposition study by Reed et al. (2009) there were no significant main effects of burning on root decomposition; however, low precipitation may have masked the effects of burning on decomposition for that study. Other studies have compared belowground decomposition in areas of contrasting burning treatments and have found that wood decomposed significantly faster in annually burned tallgrass prairie compared to unburned prairie (Reed et al., 2005; O'Lear et al., 1996). Faster decomposition in annually burned prairie soil could be due to the indirect effects of burning on the soil community composition or to the direct effects on soil conditions (i.e., heat, moisture, nutrients), which would impact decomposition processes (O'Lear et al., 1996). For example, relative to unburned tallgrass prairie soils, the soil conditions of frequently burned areas are often N-limited (Blair, 1997; Ojima et al., 1994), causing microbes to scavenge for N before beginning decomposition (Soong and Cotrufo, 2015; Craine et al., 2007). N mining by microbes in N-limited areas has been shown to increase decomposition rates in other areas (Craine et al., 2007).
Corroborating part of our third hypothesis, we found that, overall, a
significantly higher amount of
We also hypothesized that root C would be incorporated more quickly for AB. Yet, despite the overall greater amount of root C incorporation by AB, microbes and nematodes both immediately incorporated root C for both treatments (Fig. 4b and c). There was a slight lag in microbial uptake of root litter C for AB, but not for IB (Fig. 4b). This lag could correspond to the time microbes needed to scavenge N in the N-limited AB soil before commencing root decomposition (Manzoni et al., 2012). Yet, through time, evidence exists for greater cycling of root litter C to the microbivore nematodes of the AB food web. The root-litter-derived nematode-C pool was significantly greater in the AB treatment at 35 and 90 days after root addition. This accumulation of C in nematodes indicates a greater or faster flow of root litter C from the microbes to their nematode consumers. Others have suggested that most energy from detritus flows to microbes and only a negligible amount of energy flows to the higher trophic levels of the soil food web (Setälä, 2005). Our study opposes this view, as we show that per gram of soil, nematodes can hold as much as half of root-litter-derived C as microbes do (Fig. 4b and c).
Our results provide evidence that frequent fire affects decomposition
processes and adds a temporal dynamic of C flow through the soil food web.
We have shown that decomposing roots are an important C source for microbes
and nematodes in this tallgrass prairie soil.
This project was funded by the National Science Foundation under grant no. 0918482.
We are grateful to the Konza Prairie LTER site for making this
research possible. We thank the Wall Lab, especially K. Ivanovich and
E. Bernier, for assistance with work in the field and laboratory. We thank the
staff of Colorado State University's EcoCore Analytical Facility
(