Carbon and nitrogen cycling during old-field succession: Constraints on plant and microbial biomass

Soil C and N dynamics were studied in a sequence of old fields of increasing age to determine how these biogeochemical cycles change during secondary succession. In addition, three different late-successional forests were studied to represent possible "steady state" conditions. Surface soil samples collected from the fields and forests were analyzed for total C, H 2 O-soluble C, total N, potential net N mineralization, potential net nitrification, and microbial biomass. Above-and belowground plant biomass was estimated within each of the old field sites.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42472/1/10533_2004_Article_BF00002062.pd

Nitrogen mineralization, nitrification and denitrification in upland and wetland ecosystems

Nitrogen mineralization, nitrification, denitrification, and microbial biomass were evaluated in four representative ecosystems in east-central Minnesota. The study ecosystems included: old field, swamp forest, savanna, and upland pin oak forest. Due to a high regional water table and permeable soils, the upland and wetland ecosystems were separated by relatively short distances (2 to 5 m). Two randomly selected sites within each ecosystem were sampled for an entire growing season. Soil samples were collected at 5-week intervals to determine rates of N cycling processes and changes in microbial biomass. Mean daily N mineralization rates during five-week in situ soil incubations were significantly different among sampling dates and ecosystems. The highest annual rates were measured in the upland pin oak ecosystem (8.6 g N m −2 yr −1 ), and the lowest rates in the swamp forest (1.5 g N m −2 yr −1 ); nitrification followed an identical pattern. Denitrification was relatively high in the swamp forest during early spring (8040 μg N 2 O−N m −2 d −1 ) and late autumn (2525 μg N 2 O−N m −2 d −1 ); nitrification occurred at rates sufficient to sustain these losses. In the well-drained uplands, rates of denitrification were generally lower and equivalent to rates of atmospheric N inputs. Microbial C and N were consistently higher in the swamp forest than in the other ecosystems; both were positively correlated with average daily rates of N mineralization. In the subtle landscape of east-central Minnesota, rates of N cycling can differ by an order of magnitude across relatively short distances.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47791/1/442_2004_Article_BF00320810.pd

Kinetics of Microbial Respiration and Nitrogen Mineralization in Great Lakes Forests

Recent attention has focused on organic matter storage in forested ecosystems because climate change could potentially alter this process. Labile organic matter pools are especially important because they may be most strongly influenced by changes in soil temperature and water availability. We measured rates at which C and N were released from labile organic matter within the forest floor and mineral soil of jack pine (Pinus banksiana Lambert), red pine (P. resinosa Aiton), balsam fir [Abies balsamea (L.) Miller], sugar maple (Acer saccharum Marshall), and quaking aspen (Populus tremuloides Michaux) forests. Forest floor and mineral soil samples were assayed for microbial respiration and N mineralization using a long‐term (32 wk at 35 °C) laboratory incubation. Cumulative amounts of respired C and mineralized N were fit to first‐order rate equations; pools and rate constants were compared among forests. Labile (respired) C pools in forest floor ranged from 67 (jack pine) to 92 g C m−2 (sugar maple), four to six times less than that measured in mineral soil. Rate constants for microbial respiration were statistically different among forest types, but means ranged narrowly in forest floor (0.269–0.299 wk−1) and mineral soil (0.303–0.350 wk−1). Labile (mineralized) N pools ranged from 2.2 (red pine) to 4.1 g N m−2 (sugar maple) in forest floor, an order of magnitude less than those in mineral soil. Rate constants for N mineralization varied from 0.326 to 0.556 wk−1 in forest floor and from 0.043 to 0.069 wk−1 in mineral soil. Regional climatic variables were weakly correlated with labile C and N pools and with rate constants. Annual in situ estimates of microbial respiration and N mineralization were far less than respired C and mineralized N pools, suggesting that only a fraction of labile soil organic matter is annually metabolized within these forests. Local climate, rather than the chemistry of labile organic matter, appears to be an important factor constraining the annual in situ flux of C and N from this pool.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152881/1/saj2sssaj199303615995005700040037x.pd