Daniel P. Rasse

Persistence of soil organic matter as an ecosystem property

Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily—and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.

Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation

Understanding the origin of the carbon (C) stabilised in soils is crucial in order to device management practices that will foster Caccumulation in soils. The relative contributions to soilC pools of roots vs. shoots is one aspect that has been mostly overlooked, although it appears a key factor that drives the fate of plant tissueC either as mineralized CO2 or as stabilized soil organic matter (SOM). Available studies on the subject consistently indicate that rootC has a longer residence time in soil than shootC. From the few studies with complete datasets, we estimated that the mean residence time in soils of root-derived C is 2.4times that of shoot-derived C. Our analyses indicate that this value is biased neither by an underestimation of root contributions, as exudation was considered in the analysis, nor by a priming effect of shoot litter on SOM. Here, we discuss the main SOM stabilisation mechanisms with respect to their ability to specifically protect root-derived SOM. Comparing in situ and incubation experiments suggests that the higher chemical recalcitrance of root tissues as compared to that of shoots is responsible for only a small portion, i.e. about one fourth, of the difference in mean residence time in soils of root-derived vs. shoot-derivedC. This suggests that SOM protection mechanisms other than chemical recalcitrance are also enhanced by root activities: (1)physico-chemical protection, especially in deeper horizons, (2)micrometer-scale physical protection through myccorhiza and root-hair activities, and (3)chemical interactions with metal ions. The impact of environmental conditions within deeper soil layers on rootC stabilisation appear difficult to assess, but is likely, if anything, to further increase the ratio between the mean residence time of root vs. shootC in soils. Future advances are expected from isotopic studies conducted at the molecular level, which will help unravel the fate of individual shoot and root compounds, such as cutins and suberins, throughout soil profiles.

Root recolonization of previous root channels in corn and alfalfa rotations

Distribution of root systems through soils and recolonization of root channels by successive crops are fundamental, though difficult to study, processes of soil ecology. This article reports a minirhizotron (MR) study of corn and alfalfa root systems throughout the soil profile of Kalamazoo loam (fine-loamy, mixed, mesic Typic Hapludalf) monolith lysimeters for a three-year succession of corn, alfalfa and corn. Multiple-date comparisons within and between years were conducted to estimate total root densities in each soil horizon. Root recolonization was assessed by comparing every video frame of paired minirhizotrons, from recordings conducted one growing season apart. Distributions of corn root systems were modified by tillage practices. In 1994, root populations of corn in the Bt1 horizon peaked 75–90 days after planting (DAP). Numbers of corn roots per m2 in the Bt1 horizon were consistently higher for no-tillage (NT) than for conventional tillage (CT) lysimeters, in 1994 and 1996. Distribution of alfalfa roots within the soil profile was not significantly modified by tillage. However, alfalfa root decomposition rates responded to conventional and no-tillage practices and were specific for each soil horizon. Corn root systems growing in soils previously cropped with alfalfa presented similar patterns of root distribution by horizons as that of the previous alfalfa crop. Successive corn root systems did not display similar distribution patterns throughout the soil profile from one growing season to the next. Proportions of roots of the current crop recolonizing root induced macropores (RIMs) of the previous crop averaged 18% for corn after corn, 22% for alfalfa after corn and 41% for corn after alfalfa, across Bt horizons and tillage treatments. In conclusion, distribution of corn root systems appeared to be modified by tillage practices and root recolonization of RIMs was controlled by the preceding crop.

Surface Properties and Chemical Composition of Corncob and Miscanthus Biochars: Effects of Production Temperature and Method

Biochar properties vary, and characterization of biochars is necessary for assessing their potential to sequester carbon and improve soil functions. This study aimed at assessing key surface properties of agronomic relevance for products from slow pyrolysis at 250-800 °C, hydrothermal carbonization (HTC), and flash carbonization. The study further aimed at relating surface properties to current characterization indicators. The results suggest that biochar chemical composition can be inferred from volatile matter (VM) and is consistent for corncob and miscanthus feedstocks and for the three tested production methods. High surface area was reached within a narrow temperature range around 600 °C, whereas cation exchange capacity (CEC) peaked at lower temperatures. CEC and pH values of HTC chars differed from those of slow pyrolysis biochars. Neither CEC nor surface area correlated well with VM or atomic ratios. These results suggest that VM and atomic ratios H/C and O/C are good indicators of the degree of carbonization but poor predictors of the agronomic properties of biochar.

Modelling short-term CO2 fluxes and long-term tree growth in temperate forests with ASPECTS

The net ecosystem exchange (NEE) of CO2 between temperate forests and the atmosphere governs both carbon removal from the atmosphere and forest growth. In recent years, many experiments have been conducted to determine temperate forest NEE. These data have been used by forest modellers to better understand the processes that govern CO2 fluxes, and estimate the evolution of these fluxes under changing environmental conditions. Nevertheless, it is not clear whether models capable of handling short-term processes, which are mostly source-driven, can provide an accurate estimate of long-term forest growth, which is potentially more influenced by sink- and phenology-related processes. To analyse the interactions between short- and long-term processes, we developed the ASPECTS model, which predicts long-term forest growth by integrating, over time, hourly NEE estimates. Validation data consisting of measurements of NEE by eddy-covariance and forest carbon reservoir estimates were obtained from mixed deciduous and evergreen experimental forests located in Belgium. ASPECTS accurately estimated both: (1) the NEE fluxes for several years of data; and (2) the amount of carbon contained in stems, branches, leaves, fine and coarse roots. Our simulations demonstrated that: (1) NEE measurements in Belgian forests are compatible with forest growth over the course of the 20th century; and (2) that forest history and long-term processes need to be considered for accurate simulation of short-term CO2 fluxes.

Effect of elevated CO2 on carbon pools and fluxes in a brackish marsh

The effects of long-term exposure to elevated atmospheric CO2 (ambient + 340 ppmv) on carbon cycling were investigated for two plant communities in a Chesapeake Bay brackish marsh, one dominated by the C3 sedgeSchoenplectus americanus and the other by the C4 grassSpartina patens. Elevated CO2 resulted in a significant increase in porewater concentrations of DIC at 30 cm depth (p < 0.1). The CO2 treatment also yielded increases in DOC (15 to 27%) and dissolved CH4 (12–18%) in the C3 marsh (means for several depths over the period of June 1998 and June 1999), but not at a significant level. Elevated CO2 increased mean ecosystem emissions of CO2 (34–393 g C m−2 yr−1) and CH4 (0.21–0.40 g C m−2 yr−1) in the C3 community, but the effects were only significant on certain dates. For example, CO2 enrichment increased C export to the atmosphere in the C3 community during one of two winter seasons measured (p = 0.09). In the C4 community, gross photosynthesis responded relatively weakly to elevated CO2 (18% increase, p > 0.1), and the concomitant effects on dissolved carbon concentrations, respiration, and CH4 emissions were small or absent. We concluded that elevated CO2 has the potential to increase dissolved inorganic carbon export to estuaries.

Predicting transpiration from forest stands in Belgium for the 21st century

Canopy transpiration is a major element of the hydrological cycle of temperate forests. Levels of water stress during the 21st century will be largely controlled by the response of canopy transpiration to changing environmental conditions. One year of transpiration measurement in two stands (Quercus robur L. and Fagus sylvatica L.) was used to calibrate the ASPECTS model on a(1) and D-0, two parameters of a modified version of Leunings equation of stomatal conductance. A second year of data was used to validate the model. The results indicate a higher sensitivity of g(sc), to vapour pressure deficit (DS) in oak than in beech (D-0 (oak) < D-0 (beech)). To simulate future forest transpiration, site specific weather data sets were constructed from GCM outputs, spatially and temporally downscaled with local climatic data. Temperature increase between the end of the 20th and 21st centuries was predicted to be 2.8 degreesC in the beech stand and 3.1 degreesC in the oak stand. Based solely on temperature change, ASPECTS predicted an increase in transpiration of 17% in the beech and 6% in the oak stand, the difference being due to variation in local climate and the sensitivity of both species to D-s. Based solely on increased atmospheric CO2 (355 ppm in 1990 to 700 ppm in 2100), ASPECTS predicted that transpiration would decrease by 22% in beech and 19% in oak. With the combined scenarios of climatic change and increased atmospheric CO2, ASPECTS showed a decrease of 7% in transpired water in the oak stand and only 4% in the beech stand, which are not significant differences from zero. Consequently, water stress should not increase in either stand during the 21st century

TRAP: a modelling approach to below-ground carbon allocation in temperate forests

Tree root systems, which play a major role in below-ground carbon (C) dynamics, are one of the key research areas for estimating long-term C cycling in forest ecosystems. In addition to regulating major C fluxes in the present conditions, tree root systems potentially hold numerous controls over forest responses to a changing environment. The predominant contribution of tree root systems to below-ground C dynamics has been given little emphasis in forest models. We developed the TRAP model, i.e. Tree Root Allocation of Photosynthates, to predict the partitioning of photosynthates between the fine and coarse root systems of trees among series of soil layers. TRAP simulates root system responses to soil stress factors affecting root growth. Validation data were obtained from two Belgian experimental forests, one mostly composed of beech (Fagus sylvatica L.) and the other of Scots pine (Pinus sylvestris L.). TRAP accurately predicted (R = 0.88) night-time CO2 fluxes from the beech forest for a 3-year period. Total fine root biomass of beech was predicted within 6% of measured values, and simulation of fine root distribution among soil layers was accurate. Our simulations suggest that increased soil resistance to root penetration due to reduced soil water content during summer droughts is the major mechanism affecting the distribution of root growth among soil layers of temperate Belgian forests. The simulated annual rate of C input to soil litter due to the fine root turnover of the Scots pine was 207 g C m−2 yr−1. The TRAP model predicts that fine root turnover is the single most important source of C to the temperate forest soils of Belgium.

Degradation of cultivated peat soils in northern norway based on field scale CO2, N2O and CH4 emission measurements

Abstract Peat soils cover 2 – 3% of the total global land area, and store approximately one-third of the soil carbon and as much carbon as stored in the atmosphere. Conversion of natural peatland ecosystems into arable land dramatically increases soil organic matter mineralization, notably because of the required drainage operations. In the present paper, greenhouse gas emissions from cultivated peat soils were measured in Northern Norway. The results show that CO2 emissions increased with air temperature, while this effect was not clearly observed for N2O and CH4 emissions. Estimated net emissions of greenhouse gases from a pipe-drained peatland, expressed in kg of CO2 equivalent m−2 yr−1, were about 2.20 kg for CO2, 0.03 kg for CH4 and 0.13 kg for N2O, and the net carbon loss was about 0.6 kg C m−2 yr−1. Therefore, carbon loss is akin to soil degradation when effects on climate gas emissions are taken into consideration. The results demonstrate the importance of CO2 emissions from northern cultivated peatlands, which were about 17 times higher than those of N2O on a CO2-base equivalence.

Permafrost Distribution Drives Soil Organic Matter Stability in a Subarctic Palsa Peatland

Palsa peatlands, permafrost-affected peatlands characteristic of the outer margin of the discontinuous permafrost zone, form unique ecosystems in northern-boreal and arctic regions, but are now degrading throughout their distributional range due to climate warming. Permafrost thaw and the degradation of palsa mounds are likely to affect the biogeochemical stability of soil organic matter (that is, SOM resistance to microbial decomposition), which may change the net C source/sink character of palsa peatland ecosystems. In this study, we have assessed both biological and chemical proxies for SOM stability, and we have investigated SOM bulk chemistry with mid-infrared spectroscopy, in surface peat of three distinct peatland features in a palsa peatland in northern Norway. Our results show that the stability of SOM in surface peat as determined by both biological and chemical proxies is consistently higher in the permafrost-associated palsa mounds than in the surrounding internal lawns and bog hummocks. Our results also suggest that differences in SOM bulk chemistry is a main factor explaining the present SOM stability in surface peat of palsa peatlands, with selective preservation of recalcitrant and highly oxidized SOM components in the active layer of palsa mounds during intense aerobic decomposition over time, whereas SOM in the wetter areas of the peatland remains stabilized mainly by anaerobic conditions. The continued degradation of palsa mounds and the expansion of wetter peat areas are likely to modify the bulk SOM chemistry of palsa peatlands, but the effect on the future net C source/sink character of palsa peatlands will largely depend on moisture conditions and oxygen availability in peat.