The effects of land use change on soil carbon stocks are of concern in the context of international policy agendas on greenhouse gas emissions mitigation. This paper reviews the literature for the influence of land use changes on soil C stocks and reports the results of a meta analysis of these data from 74 publications. The meta analysis indicates that soil C stocks decline after land use changes from pasture to plantation (−10%), native forest to plantation (−13%), native forest to crop (−42%), and pasture to crop (−59%). Soil C stocks increase after land use changes from native forest to pasture (+ 8%), crop to pasture (+ 19%), crop to plantation (+ 18%), and crop to secondary forest (+ 53%). Wherever one of the land use changes decreased soil C, the reverse process usually increased soil carbon and vice versa. As the quantity of available data is not large and the methodologies used are diverse, the conclusions drawn must be regarded as working hypotheses from which to design future targeted investigations that broaden the database. Within some land use changes there were, however, sufficient examples to explore the role of other factors contributing to the above conclusions. One outcome of the meta analysis, especially worthy of further investigation in the context of carbon sink strategies for greenhouse gas mitigation, is that broadleaf tree plantations placed onto prior native forest or pastures did not affect soil C stocks whereas pine plantations reduced soil C stocks by 12–15%.

Soil carbon stocks and land use change: a meta analysis

Framework Convention on Climate Change

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol1. Several studies suggest that the terrestrial biosphere is gaining carbon2,3,4,5,6,7,8, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha-1 yr-1, with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.

Respiration as the main determinant of carbon balance in European forests

Legumes, broadly defined by their unusual flower structure, podded fruit, and the ability of 88% of the species examined to date to form nodules with rhizobia ([de Faria et al., 1989][1]), are second only to the Graminiae in their importance to humans. The 670 to 750 genera and 18,000 to 19,000

Legumes: Importance and Constraints to Greater Use

Forest soils and carbon sequestration

Forests with nitrogen-fixing trees (N‐fixers) typically accumulate more carbon (C) in soils than similar forests without N‐fixing trees. This difference may develop from fundamentally different processes, with either greater accumulation of recently fixed C or reduced decomposition of older soil C. We compared the soil C pools under N‐fixers with Eucalyptus (non‐N‐fixers) at four tropical sites: two sites on Andisol soils in Hawaii and two sites on Vertisol and Entisol soils in Puerto Rico. Using stable carbon isotope techniques, we tracked the loss of the old soil organic C from the previous C4 land use (SOC4) and the gain of new soil organic C from the C3, N‐fixer, and non‐N‐fixer plantations (SOC3). Soils beneath N‐fixing trees sequestered 0.11 0.07 kg m 2 y 1 (mean one standard error) of total soil organic carbon (SOCT) compared with no change under Eucalyptus (0.00 0.07 kg m 2 y 1 ; P 0.02). About 55% of the greater SOC T sequestration under the N‐fixers resulted from greater retention of old SOC4, and 45% resulted from greater accretion of new SOC3. Soil N accretion under the N‐fixers explained 62% of the variability of the greater retention of old SOC4 under the N‐fixers. The greater retention of older soil C under N‐fixing trees is a novel finding and may be important for strategies that use reforestation or afforestation to offset C emissions.

https://www.fs.fed.us/global/iitf/pubs/ja_iitf_2002_resh001.pdf

Greater Soil Carbon Sequestration under Nitrogen-fixing Trees Compared with Eucalyptus Species

Tree plantations are an important component of tropical landscapes, providing wood, fuel, and perhaps carbon (C) sequestration. Primary production in wet tropical plantations is typically nutrient limited. In some Hawaiian Eucalyptus plantations, nitrogen (N) limitations to production are alleviated by intercropping N-fixing Albizia trees that may decrease available phosphorus (P). Thus, sustainable productivity and C sequestration may depend on species composition. We measured soil N and P availability and ecosystem N and C sequestration in a 17-yr-old replicated replacement series of Eucalyptus and Albizia in Hawaii. Species composition included pure plots of each species and four proportions of mixtures. Soil N availability increased with the proportion of Albizia in the plot, but soil P availability declined. Aboveground tree C accumulation showed a synergistic response to increasing percentage of Albizia, with the mixed stands having more tree C than pure stands of Eucalyptus or Albizia. In the top 50 cm of soil, total N and C increased linearly with percentage of Albizia. Stands with the highest percentage of Albizia had 230 g/m2 more soil N and 2000 g/m2 more soil C than stands without Albizia. Stable C isotope analyses showed that increased soil C resulted from differences in both tree-derived C and “old” sugarcane-derived C. Deeper soil C (50–100 cm) was a substantial fraction (0.36) of total soil C but did not vary among treatments. Our results demonstrate that tree species effects on nutrient and C dynamics are not as simple as monocultures suggest. Mixed-species afforestation increased tree and soil C accrual over 17 years, and N inputs may increase soil C storage by decreasing decomposition.

Nutrient and carbon dynamics in a replacement series of Eucalyptus and Albizia trees

Intensively managed plantations of trees occupy vast areas of the tropics The productivity of these forests depends strongly on nutrient supply, and nutrient supply may change rapidly under intensive management regimes We documented changes in a Hawaiian soil after 32 mo of development of a plantation of eucalyptus [Eucalyptus saligna (Sm)] Soil C did not change significantly (average = 23 g C m -2 yr -1 to 30 cm; 95% confidence -139 to +93 g C m -2 yr ') This lack of change in soil C resulted from a rapid loss of older soil C derived from sugarcane (-191 g C m 2 yr -1 ) and a rapid gain of new soil C from eucalyptus (160 g C m -2 yr -1 ) Soil N declined by 19 g N m 2 yr -1 (P = 008), despite fertilizer additions of 31 to 70 g m 2  Large reductions in exchangeable Ca and Mg probably resulted from dissolution and leaching of residual lime from prior agricultural management We conclude that intensive sampling regimes may detect relatively small changes in tropical forest soils, and that expectations of C accumulation in soils following afforestation may need to be reconsidered

Rapid changes in soils following eucalyptus afforestation in Hawaii

This study develops a feasible method for evaluating coarse root biomass (roots >2 mm diameter) of well established plantations of eucalypts and then examines coarse root biomass variability across tree age and size, fertilization treatment, species and site for Eucalyptus globulus and E. nitens in Tasmania, Australia. The most efficient sampling protocol consisted of rootball excavation and soil coring for bulk coarse roots, which when compared with total tree excavation estimated total coarse root biomass contained inside the sampled area to within 10%. Across all treatments, an average of 76% of the coarse root biomass was located within the rootball. The majority (>65%) of the coarse roots outside the rootball were located in the surface 20 cm of soil. When size class distribution was examined, 75% of coarse root biomass was found to occur in the larger (20+ mm) diameter size class, a size class that displayed considerable spatial heterogeneity. At the stand level, coarse root biomass ranged from 2.18 to 7.38 kg m−2 depending primarily on tree size but also on fertilization treatment, species and site. It is estimated that allocation to coarse root biomass production was around 0.2 kg m−2 year−1 (around 6% of estimated NPP) for the E. nitens stands examined in this study and around 1 kg m−2 year−1 (around 20% of estimated NPP) for the E. globulus stand examined. Robust relationships using above-ground parameters could be used to predict coarse root biomass regardless of fertilization or site, but species changed the relationship.

Coarse root biomass for eucalypt plantations in Tasmania, Australia: sources of variation and methods for assessment

A decline in wood production and above-ground net primary production (ANPP) following an early maximum is a widely observed feature of forest development. Why should relatively young, seemingly vigorous even-aged forests undergo this decrease in production? The authors measured above-ground net primary production and below-ground carbon allocation in an age sequence of lodgepole pine forest in south-central Wyoming spanning 260 yr of forest development. ANPP and total root carbon allocation (TRCA) are examined to determine if there is an increase in TRCA of sufficient magnitude to offset the observed decrease in ANPP in developing lodgepole forests. Also, they examined changes in canopy structure, leaf area efficiency (E), and above-ground carbon allocation patterns to determine their potential role in the age-related decrease in ANPP. ANPP declined by 50% between peak production of 192 gC m{sup {minus}2}yr{sup {minus}1} at age 30 to 92 gC m{sup {minus}2}yr{sup {minus}1} at age 260, largely as a decrease in stem wood production. TRCA was similar for forests from 30 to 100 yr old, but was significantly less in 260-yr-old forest at 391 gC m{sup {minus}2}yr{sup {minus}1} than in younger lodgepole forests at 516 gC m{sup {minus}2}yr{sup {minus}1}. Declining ANPP was not due to an increase inmore » carbon allocation to below-ground production. Net primary production (NPP), the total of above-ground and below-ground NPP, declined by 36% from 450 gC m{sup {minus}2}yr{sup {minus}1} at age 30 to 288 gC m{sup {minus}2}yr{sup {minus}1} at age 260. This progressive decline in NPP was associated with changes in stand structure that occur during stand development, including decreased leaf area, shifting carbon allocation, and decline in leaf area efficiency. The results suggest that a combination of several factors, acting in concert, produce the observed decline in ANPP associated with the development of even-ages lodgepole pine forests.« less

Sigrid C. Resh

Papers

Greater Soil Carbon Sequestration under Nitrogen-fixing Trees Compared with Eucalyptus Species

Nutrient and carbon dynamics in a replacement series of Eucalyptus and Albizia trees

Rapid changes in soils following eucalyptus afforestation in Hawaii

Coarse root biomass for eucalypt plantations in Tasmania, Australia: sources of variation and methods for assessment

Age-related changes in production and below-ground carbon allocation in Pinus contorta forests