Daniel Markewitz

Rapid accumulation and turnover of soil carbon in a re-establishing forest

Present understanding of the global carbon cycle is limited by uncertainty over soil-carbon dynamics,,,,,. The clearing of the worlds forests, mainly for agricultural uses, releases large amounts of carbon to the atmosphere (up to 2× 1015 g yr−1), much of which arises from the cultivation driving an accelerated decomposition of soil organic matter,,,. Although the effects of cultivation on soil carbon are well studied, studies of soil-carbon recovery after cultivation are limited,,,,,,,. Here we present a four-decade-long field study of carbon accumulation by pine ecosystems established on previously cultivated soils in South Carolina, USA. Newly accumulated carbon is tracked by its distinctive 14C signature, acquired around the onset of forest growth from thermonuclear bomb testing that nearly doubled atmospheric 14CO2 in the 1960s. Field data combined with model simulations indicate that the young aggrading forest rapidly incorporated bomb radiocarbon into the forest floor and the upper 60 cm of underlying mineral soil. By the 1990s, however, carbon accumulated only in forest biomass, forest floor, and the upper 7.5 cm of the mineral soil. Although the forest was a strong carbon sink, trees accounted for about 80%, the forest floor 20%, and mineral soil <1%, of the carbon accretion. Despite high carbon inputs to the mineral soil, carbon sequestration was limited by rapid decomposition, facilitated by the coarse soil texture and low-activity clay mineralogy.

The effects of partial throughfall exclusion on canopy processes, aboveground production, and biogeochemistry of an Amazon forest

(1) Moist tropical forests in Amazonia and elsewhere are subjected to increasingly severe drought episodes through the El Nino-Southern Oscillation (ENSO) and possibly through deforestation-driven reductions in rainfall. The effects of this trend on tropical forest canopy dynamics, emissions of greenhouse gases, and other ecological functions are potentially large but poorly understood. We established a throughfall exclusion experiment in an east-central Amazon forest (Tapajos National Forest, Brazil) to help understand these effects. After 1-year intercalibration period of two 1-ha forest plots, we installed plastic panels and wooden gutters in the understory of one of the plots, thereby excluding � 890 mm of throughfall during the exclusion period of 2000 (late January to early August) and � 680 mm thus far in the exclusion period of 2001 (early January to late May). Average daily throughfall reaching the soil during the exclusion period in 2000 was 4.9 and 8.3 mm in the treatment and control plots and was 4.8 and 8.1 mm in 2001, respectively. During the first exclusion period, surface soil water content (0-2 m) declined by � 100 mm, while deep soil water (2-11 m) was unaffected. During the second exclusion period, which began shortly after the dry season when soil water content was low, surface and deep soil water content declined by � 140 and 160 mm, respectively. Although this depletion of soil water provoked no detectable increase in leaf drought stress (i.e., no reduction in predawn leaf water potential), photosynthetic capacity declined for some species, the canopy thinned (greater canopy openness and lower leaf area index) during the second exclusion period, stem radial growth of trees <15 m tall declined, and fine litterfall declined in the treatment plot, as did tree fruiting. Aboveground net primary productivity (NPP) (stemwood increment and fine litter production) declined by one fourth, from 15.1 to 11.4 Mg ha � 1 yr � 1 , in the treatment plot and decreased slightly, from 11.9 to 11.5 Mg ha � 1 yr � 1 , in the control plot. Stem respiration varied seasonally and was correlated with stem radial growth but showed no treatment response. The fastest response to the throughfall exclusion, and the surface soil moisture deficits that it provoked, was found in the soil itself. The treatment reduced N2O emissions and increased CH4 consumption relative to the control plot, presumably in response to the improved soil aeration that is associated with soil drying. Our hypothesis that NO emissions would increase following exclusion was not supported. The conductivity and alkalinity of water percolating through the litter layer and through the mineral soil to a depth of 200 cm was higher in the treatment plot, perhaps because of the lower volume of water that was moving through these soil layers in this plot. Decomposition of the litter showed no difference between plots. In sum, the small soil water reductions provoked during the first 2 years of partial throughfall exclusion were sufficient to lower aboveground NPP, including the stemwood increment that determines the amount of carbon stored in the

How Deep Is Soil?Soil, the zone of the earth's crust that is biologically active, is much deeper than has been thought by many ecologists

arth is a most remarkable planet but not only because of its prodigious life, vast oceans, and oxygen-enriched atmosphere. Earth is remarkable because of its soil. Soil is the biologically excited layer of the earths crust. It is an organized mixture of organic and mineral matter. Soil is created by and responsive to organisms, climate, geologic processes, and the chemistry of the aboveground atmosphere. Soil is the rooting zone for terrestrial plants and the filtration medium that influences the quality and quantity of Earths waters. Soil supports the nearly unexplored communities of microorganisms that decompose organic matter and recirculate many of the biospheres chemical elements. Ecologists consider soil to be the central processing unit of the earths environment (Sanchez 1994). One of the most significant outcomes of biological evolution has been the coevolution of soil and terrestrial ecosystems. This coevolution was initiated during the Devonian era, approximately 350 million years ago. Plants spread across upland continental regions during the explosion of life that led directly

SOIL CHEMICAL CHANGE DURING THREE DECADES IN AN OLD-FIELD LOBLOLLY PINE (PINUS TAEDA L.) ECOSYSTEM'

The ability of soil to sustain its supply of nutrients to a growing forest is controlled by a complex of biogeochemical processes. Forest soil data are notably absent, however, that describe sustained nutrient supply or nutrient depletion. The objective of this study was to evaluate how exchangeable nutrient cations of a previously cultivated Ultisol responded to the first three decades of pine forest development. On six occasions during the three decades, the upper 0.6 m of soil was sampled from eight permanent plots and chemically analyzed with the same procedures. During this period, KCl-exchangeable acidity (as positive charges of adsorbed H and Al ions) increased by 37.3 kmol,/ha in the upper 0.6 m of soil and positive charges of exchangeable Ca and Mg were depleted by 34.8 and 8.9 kmolc/ha (by 696 and 108 kg/ha), whereas, exchangeable K was reduced by only 0.5 kmolc/ha (19 kg/ha). Depletion of soil exchangeable Ca was on the same order of magnitude as Ca removals (i.e., Ca accumulation in biomass and forest floor plus that lost in soil leaching). Removals of soil Mg also appeared to outpace resupply from recycling, atmospheric deposition, and mineral weathering, but not to the same degree as Ca. Over the three decades, soil leaching loss of these divalent cations (from 0.6 m depth) appeared equal to cation accumulation in biomass plus forest floor, with sulfate balancing about half these cations in leachates. In contrast to Ca and Mg, total K removals from the soil exceeded reductions in soil exchangeable K by nearly 20-fold. Exchangeable K was well buffered in surface mineral soils apparently due to a combination of biological recycling via leaching of canopies and forest floor plus mineral weathering release. These nutrient dynamics may be common to many nutrient-demanding forest ecosystems supported by soils with low activity kandic or oxic horizons. Such soils (Ultisols and Oxisols) occur on many hundreds of millions of hectares in temperate and tropical zones.

ECOLOGICAL RESEARCH IN THE LARGE-SCALE BIOSPHERE– ATMOSPHERE EXPERIMENT IN AMAZONIA: EARLY RESULTS

The Large-scale Biosphere-Atmosphere Experiment in Amazonia (LBA) is a multinational, interdisciplinary research program led by Brazil. Ecological studies in LBA focus on how tropical forest conversion, regrowth, and selective logging influence carbon storage, nutrient dynamics, trace gas fluxes, and the prospect for sustainable land use in the Amazon region. Early results from ecological studies within LBA emphasize the var- iability within the vast Amazon region and the profound effects that land-use and land- cover changes are having on that landscape. The predominant land cover of the Amazon region is evergreen forest; nonetheless, LBA studies have observed strong seasonal patterns in gross primary production, ecosystem respiration, and net ecosystem exchange, as well as phenology and tree growth. The seasonal patterns vary spatially and interannually and evidence suggests that these patterns are driven not only by variations in weather but also by innate biological rhythms of the forest species. Rapid rates of deforestation have marked the forests of the Amazon region over the past three decades. Evidence from ground-based surveys and remote sensing show that substantial areas of forest are being degraded by logging activities and through the collapse of forest edges. Because forest edges and logged forests are susceptible to fire, positive feedback cycles of forest degradation may be initiated by land-use-change events. LBA studies indicate that cleared lands in the Amazon, once released from cultivation or pasture usage, regenerate biomass rapidly. However, the pace of biomass accumulation is dependent upon past land use and the depletion of nutrients by unsustainable land-management practices. The challenge for ongoing research within LBA is to integrate the recognition of diverse patterns and processes into general models for prediction of regional ecosystem function.

NUTRIENT LOSS AND REDISTRIBUTION AFTER FOREST CLEARING ON A HIGHLY WEATHERED SOIL IN AMAZONIA

Over the past three decades, tropical forest clearing and burning have greatly altered the Amazonian landscape by increasing the cover of pastures and secondary forests. The alteration of biogeochemical processes on these lands is of particular interest on highly weathered Oxisols that cover large areas in the region because of concerns regarding possible nutrient limitation in agricultural land uses and during forest regrowth. The ob- jectives of this study were to quantify (1) the reaccumulation of nutrients in biomass of secondary land uses, (2) changes in soil nutrient contents, (3) internal nutrient cycles, and (4) input-output budgets for the landscape mosaic. Nutrient stocks and fluxes were quantified from 1996 to 1998 in mature forest, 19-yr- old secondary forest, degraded pastureland, and managed pastureland in the Brazilian state of Para ´. Mature forests contain 130 Mg C/ha in aboveground biomass while secondary forest, degraded pasture, and managed pasture contain 34, 4, and 3 Mg C/ha, respectively. Reaccumulation of N, P, K, Ca, and Mg in aboveground biomass of secondary forest was 20%, 21%, 42%, 50%, and 27% of that present in mature forest, while degraded pasture contained 2%, 4%, 15%, 11%, and 6%. Managed pasture had similar accumulations as degraded pasture except for Ca (3%). Changes in soil stocks of C, N, and P were not detected among land uses, except in fertilized managed pastures, where total soil P (0-10 cm) was elevated. Conversely, Meh- lich-III-extractable P of all secondary lands were very low ( ,1 mg/g) and were 1 kg/ha less than contents (0-10 cm) in mature forest. NaOH-extractable P was present in 100-fold higher concentrations and may gradually contribute to meeting plant demands over decadal time scales. Soil cation contents (0-20 cm) were elevated in secondary lands with increases of ;85, 500, and 75 kg/ha for K, Ca, and Mg, respectively. These increases could account for a substantial portion of cation contents originally in the aboveground biomass of mature forest. The recycling of nutrients through ;9.0 Mg·ha 21

Control of cation concentrations in stream waters by surface soil processes in an Amazonian watershed.

The chemical composition of ground waters and stream waters is thought to be determined primarily by weathering of parent rock. In relatively young soils such as those occurring in most temperate ecosystems, dissolution of primary minerals by carbonic acid is the predominant weathering pathway that liberates Ca2+, Mg2+ and K+ and generates alkalinity in the hydrosphere. But control of water chemistry in old and highly weathered soils that have lost reservoirs of primary minerals (a common feature of many tropical soils) is less well understood. Here we present soil and water chemistry data from a 10,000-hectare watershed on highly weathered soil in the Brazilian Amazon. Streamwater cation concentrations and alkalinity are positively correlated to each other and to streamwater discharge, suggesting that cations and bicarbonate are mainly flushed from surface soil layers by rainfall rather than being the products of deep soil weathering carried by groundwater flow. These patterns contrast with the seasonal patterns widely recognized in temperate ecosystems with less strongly weathered soils. In this particular watershed, partial forest clearing and burning 30 years previously enriched the soils in cations and so may have increased the observed wet season leaching of cations. Nevertheless, annual inputs and outputs of cations from the watershed are low and nearly balanced, and thus soil cations from forest burning will remain available for forest regrowth over the next few decades. Our observations suggest that increased root and microbial respiration during the wet season generates CO2 that drives cation-bicarbonate leaching, resulting in a biologically mediated process of surface soil exchange controlling the streamwater inputs of cations and alkalinity from these highly weathered soils.

Soil moisture depletion under simulated drought in the Amazon: impacts on deep root uptake.

*Deep root water uptake in tropical Amazonian forests has been a major discovery during the last 15 yr. However, the effects of extended droughts, which may increase with climate change, on deep soil moisture utilization remain uncertain. *The current study utilized a 1999-2005 record of volumetric water content (VWC) under a throughfall exclusion experiment to calibrate a one-dimensional model of the hydrologic system to estimate VWC, and to quantify the rate of root uptake through 11.5 m of soil. *Simulations with root uptake compensation had a relative root mean square error (RRMSE) of 11% at 0-40 cm and < 5% at 350-1150 cm. The simulated contribution of deep root uptake under the control was c. 20% of water demand from 250 to 550 cm and c. 10% from 550 to 1150 cm. Furthermore, in years 2 (2001) and 3 (2002) of throughfall exclusion, deep root uptake increased as soil moisture was available but then declined to near zero in deep layers in 2003 and 2004. *Deep root uptake was limited despite high VWC (i.e. > 0.30 cm(3) cm(-3)). This limitation may partly be attributable to high residual water contents (theta(r)) in these high-clay (70-90%) soils or due to high soil-to-root resistance. The ability of deep roots and soils to contribute increasing amounts of water with extended drought will be limited.

The bio in aluminum and silicon geochemistry

The translocation and transformation of Al and Si are of paramount importance in the processes of primary-mineral weathering, saprolite formation and soil formation. Geochemical mass balance studies of these processes have often not considered the important role of the biota in cycling of these omnipresent soil elements. In the Calhoun Experimental Forest, SC, we found a mean annual biological uptake of Al and Si of 2.28 and 15.8 kg ha-1 yr-1, respectively, with a mean annual accumulation in aboveground biomass of 0.48 and 2.32 kg ha-1 yr-1, respectively. In the case of Al, net soil leaching from 6 m depth is zero, thus biomass accumulation of Al accounts for the only removal from the soil system. There is an additional internal system mobilization of Al of 6.6 kg ha-1 yr-1, in response to biotic inputs of dissolved organic carbon. In the case of Si, net soil leaching to groundwater is 17.26 kg ha-1 yr-1. The accumulation of Si in aboveground biomass, 2.32 kg ha-1 yr-1, and in forest floor organic matter, 11.95 kg ha-1 yr-1, augments the annual weathering release estimate of Si by an additional 82%. The inclusion of biological cycling of both essential and non-essential mineral elements is important for properly evaluating the biogeochemistry of the earths crust.

SOIL CHANGE AND CARBON STORAGE IN LONGLEAF PINE STANDS PLANTED ON MARGINAL AGRICULTURAL LANDS

An increasing area of marginal agricultural land in the coastal plain of the southeastern United States is being planted to longleaf pine (Pinus palustris Mill.). This chronosequence study in southern Georgia evaluated the effect of pine planting and the associated cessation of agricultural activity such as tillage and fertilization on soil C storage and soil nutrient stocks. Soils are Arenic or Typic Kandiudults with coarse-textured surface soils. Soil C, nutrients, and bulk density from 0 to 50 cm in planted stands 1, 3, 7, and 14 yr old, as well as soils beneath natural longleaf pine stands that were in a never tilled (NT) condition, were evaluated (n = 3 per stand age). No accumulation of soil C was apparent during the first 14 yr of pine growth. The average content of soil C in planted stands (11 ± 1 Mg/ha; mean ± 1 se) was ∼16 Mg/ha less than that in the NT soils (27 ± 4 Mg/ha). Soil total N content within planted stands also did not differ by age, although extractable NO3 declined rapidly. Despite ...