Effects of Long-Term Soil Warming on Aggregate Mass and Physical Protection of Organic Matter

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1 Effects of Long-Term Soil Warming on Aggregate Mass and Physical Protection of Organic Matter Luis Cartagena, Northwestern University ABSTRACT Global warming may induce accelerated soil organic matter (SOM) decomposition through increased soil temperature, and thus impact the C balance in soils. In a 26-year soil warming experiment in a mid-latitude hardwood forest, I documented the role played by aggregates and physical protection of organic matter in both heated and undisturbed soils in controlling SOM decay to investigate the potential consequences for the climate system. I found that long-term soil warming results in the reduction of aggregate mass, microbial biomass C, the concentration of soil C, and that the disruption of physical protection of organic matter, via crushing, accelerates SOM decay. My results indicate that aggregates help to stabilize soil carbon in New England Forest. INTRODUCTION Soil organic matter decomposition is the most important and complicated reservoir of the global carbon cycle, as it has a significant impact on atmospheric CO2 levels. Global climate change may induce an acceleration of SOM decomposition through increased soil temperatures, which can significantly impact the C balance in soils (Davidson and Janssens, 2006; Peterjohn et al., 1994). Warmer temperatures can lead microorganisms in the soil to become more active in breaking down plant matter and other materials (Metcalfe, 2017). This would increase microbial respiration rates, or CO2 efflux into the atmosphere, and result in a self-reinforcing climate feedback more warming. Soil physical structure, organic inputs and microbial activity together impact the rate at which organic matter is decomposed and stored within soils (Bach et al., 2014). Soil aggregates are groups of soil particles that bind to each other and add stability to soil physical structure. Within soil aggregates are relatively easy to decompose organic matter labile carbon that is physically protected from coming into contact with microbes and extracellular enzymes. The role played by aggregates in controlling SOM decay in New England forests has not previously been investigated and is the purpose of the current study. Specifically, I conducted a study of aggregates and decomposition in undisturbed and disturbed soils from the Harvard Forest in central Massachusetts. The disturbed soil collected from this site has been artificially heated 5 C above ambient temperature for the past 27 years as part of the soil warming experiment lead by Melillo and his colleagues. I hypothesize that long-term soil warming depletes aggregate mass and that physical disruption of aggregates via crushing accelerates respiration rates. I performed a shortterm laboratory incubation to test the relationship between the temperature sensitivity of soil respiration and physical protection by disrupting soil aggregates. My hypothesis was tested using whole soil and crushed soil subjected to three degrees of physical disruption. MATERIALS AND METHODS Sites and Sampling Soil was collected in November of 2017 from the Harvard Forest in Petersham, MA (42.5 N; 72 W). The soil is mainly sandy loam glacial till, with some alluvial and colluvial deposits; moderately to well drained in most areas; acidic, and has an average depth of 1m. The area of sampling is dominated by red maple (Acer rubrum L.) and black oak (Quercus velutina Lam.) and. Mean annual precipitation for the area is 110 cm, July mean temperature is 20 C and January mean temperature is -7 C. The Harvard Forest site contains 18 plots, each 6 m x 6 m, that are grouped into six blocks. The three plots within each block are randomly assigned to one of three treatments: (i) heated

2 plots in which the average soil temperature is continuously elevated 5 C above ambient by the use of buried heating cables; (ii) disturbance control plots that are identical to the heated plots except that they receive no electrical power; and (iii) undisturbed control plots (referred to as control in this paper) that have been left in their natural state (no cables). Thirty-six soil cores (10-cm height and 5-cm diameter) were collected from the field, three from each of the six heated plots (eighteen total) and three from each of the six disturbance control plots (eighteen total). Only the mineral horizon was used for the experiment. Analyses The field-moist soils were composited by treatment, roots and stones removed (Table 1). Samples were analyzed for carbon and nitrogen concentrations; microbial biomass C via direct extraction and the Aurora 1030 W dissolved organic carbon analyzer using per sulfate digestion; and macroaggregates (2-4mm) mass per unit total soil was determined (after sieving and using a C1 Platform Shaker at 50 rpm). The remaining soil from each treatment was air dried and divided in two. Half of the soil was left intact and half was crushed via mortar and pestle, then each prepared for incubation (Plante et al., 2009) by rewetting to field-capacity. A LICOR 6400 infrared gas analyzer (LICOR Corp., Lincoln, NE) was used to measure soil CO2 emission rates periodically for 12 days. RESULTS I measured carbon concentration in the soils via isotope analysis and learned that the mineral horizon of the Harvard Forest control soil is 6.49 percent carbon and that the heated soil is 5.29 percent carbon (Table 2). Another measurement was of microbial biomass carbon. I learned that the heated plots had roughly 50% less microbial biomass carbon relative to the control plots (Figure 1). After sieving, I learned that the control soils have 50% more aggregate mass than the heated soils (Figure 2). Figure 3 shows that at both 20 C and 25 C, temperature sensitivity of soil respiration in the intact control soil was greater than in the intact heated soil; at 15 C this relationship did not hold. Figures 4, 5, and 6 show that crushing accelerates respiration rates in both the control and heated soils. DISCUSSION AND CONCLUSIONS Experimental evidence for the physical protection of SOM in macroaggregates has come from a number of experiments that have shown increased C and N mineralization with crushing, particularly in cultivated soils (as reviewed by Balesdent et al., 2000; Six et al., 2002). The physical disruption treatment used in this experiment seems to have been sufficient to release large amounts of physically protected SOM for decomposition, as observed by increased respiration rates in the crushed soils compared to the intact soils. This suggest that crushing releases labile carbon and increases substrate availability for microbes. The lack of differences in the cumulative loss of carbon values among physical disruption treatments in the 15 C control and heated plots does not support my hypothesis that the largest respiration values would be observed in the treatment with the least physical protection, and perhaps reflects a methodological error. However, at both 20 C and 25 C, my hypothesis is supported as temperature sensitivity of soil respiration in the intact control soil was greater than in the intact heated soil. This is consistent with the acclimation phenomenon observed by Melillo and colleagues in their long-term soil warming experiment at the Harvard Forest. The acclimation phenomenon refers to the observation that at any given temperature, warmer soils will respire less than non-heated soils. Although we don t fully understand the mechanism behind thermal acclimation, we know that the heated soils from the Harvard Forest contain a lower concentration of carbon and about 50% less microbial biomass than the undisturbed soils, so we might postulate a relationship between thermal acclimation, soil carbon concentration, microbial biomass, and soil aggregation. Soil aggregation is known to reduce the decomposability of otherwise labile material by physically separating the substrate from the microbial biomass and its enzymes (Oades, 1984) While temperature dependence of total soil respiration is well established, much less is known about how that dependence affects different groups of microorganisms. Microbial biomass C data alone is clearly insufficient to determine changes in the microbial community composition and activity, and further studies in this area should be supported by community

3 structure analyses as well as substrate utilization efficiency analyses, perhaps via rrna analysis (Melillo et al., 2017). In sum, I performed several treatments on the control and heated plots of the Harvard Forest site to explore the role of aggregate mass and physical disruption in controlling soil organic matter decay. Measurement of macroaggregates (2-4mm) as percent of total soil mass showed that heated soils contain 50% less aggregates than control sols, and the incubations of soil samples with varying degrees of physical disruption showed significant differences in respiration rates. Based on these results, I conclude that physical protection of SOM is an important mechanism that drives SOM decomposition. It remains important, however, to test how SOM protection mechanisms were reduced in the heated soils and its relationship to microbial biomass and soil carbon concentration, because even fractionally small changes in temperature sensitivity under changing climate can result in globally significant changes in soil C stocks. Oades, J.M Soil organic matter and structural stability: Mechanisms and implications for management. Plant Soil 76: Peterjohn, W. T., Melillo, J. M., Steudler, P. A., Newkirk, K. M., Bowles, F. P. and Aber, J. D Responses of Trace Gas Fluxes and N Availability to Experimentally Elevated Soil Temperatures. Ecological Applications 4: Plante, A. F., J. Six, E. A. Paul, and R. T. Conant Does Physical Protection of Soil Organic Matter Attenuate Temperature Sensitivity? Soil Science Society of America Journal 73:1168. Six, J., C. Feller, K. Denef, S.M. Ogle, J.C.D. Sa, and A. Albrecht Soil organic matter, biota and aggregation in temperate and tropical soils: Effects of no-tillage. Agronomie 22: ACKNOWLEDGMENT I would like to thank my adviser, Jerry Melillo; Will Werner, Harvard Forest Project Manager, and Lois Anderson for their assistance in field sampling; Marine Biological Laboratory, for access to laboratory facilities; Rich McHorney, Jordan Stark, Alana Thurston, Em Stone for laboratory assistance. REFERENCES Bach, E. M., and K. S. Hofmockel Soil aggregate isolation method affects measures of intra-aggregate extracellular enzyme activity. Soil Biology and Biochemistry 69: Balesdent, J., C. Chenu, and M. Balabane Relationship of soil organic matter dynamics to physical protection and tillage. Soil Tillage Res. 53: Davidson, E.A., and I.A. Janssens Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature 440: Davidson, E.A., and I.A. Janssens, and Y.Q. Luo On the variability of respiration in terrestrial ecosystems: Moving beyond Q 10. Global Change Biol. 12: Melillo, J. M., S. D. Frey, K. M. Deangelis, W. J. Werner, M. J. Bernard, F. P. Bowles, G. Pold, M. A. Knorr, and A. S. Grandy Long-term pattern and magnitude of soil carbon feedback to the climate system in a warming world. Science 358: Metcalfe, D. B Microbial change in warming soils. Science 358:41 42.

4 Table 1. Summary of treatments. Experimental Design Field manipulation Treatments Temperatures Replicates Total heated, control whole soil, crushed soil 15, 20, 25 C 3 36 Table 2. Harvard Forest mineral horizon: carbon and nitrogen concentrations and isotopes. Field manipulation %C δ 13 C %N δ 15 N Disturbed control Heated Figure 1. Macroaggregates (2-4mm) as percent of total soil mass, determined via sieving. 160 Biomass (mg C/kg dry soil) Control Heated Figure 2. Microbial biomass carbon in control and heated plots.

5 Soil CO 2 Flux (μg CO 2 C kg -1 soil hr -1 ) Figure 3. Cumulative loss of carbon throughout a 12-day period across three temperatures, 15, 20, and 25 C in both the control and heated intact soils, determined via respiration rates. Figure 4. Cumulative loss of carbon throughout a 12-day period across all treatments, determined via respiration rates C intact 15 C crushed 20 C intact 20 C crushed 25 C intact 25 C crushed Time (d) Figure 5. Average control soil CO2 flux throughout a 12-day period.

6 Soil CO 2 Flux (μg CO 2 C kg -1 soil hr -1 ) C intact 15 C crushed 20 C intact 20 C crushed 25 C intact 25 C crushed Time (d) Figure 6. Average heated soil CO2 flux throughout a 12-day period.