[1] The enhancement of infiltration capacity in the presence of vegetation is well documented in arid ecosystems where it can significantly impact the water balance and vegetation spatial organization. To begin progress toward developing a theory of vegetation-infiltration interactions across a wide spectrum of climate regimes, three key questions are addressed: (1) Does vegetation also enhance infiltration capacity in mesic to hydric climates, and if so, what processes contribute to this enhancement? (2) Is there a canonical relationship between vegetation biomass and infiltration rate? and (3) How does the vegetation-infiltration feedback evolve across climatic gradients? To address these three questions, new field data examining biomass-infiltration relationships in different vegetation types in a humid climate and on loamy soils are combined with a meta-analysis of biomass-infiltration relationships from nearly 50 vegetation communities spanning a climatic gradient from hyperarid deserts to the humid tropics and representing a full spectrum of soil types. Infiltration capacity increased as a power law function of aboveground biomass in water-limited ecosystems, but vegetation biomass was not significantly correlated to infiltration capacity in humid climates. Across a climatic gradient from xeric to hydric, the slope of the power law relationship between aboveground biomass and infiltration capacity decreased.

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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 of 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 kmolc/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 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.

Reforestation of formerly cultivated land is widely understood to accumulate above- and belowground detrital organic matter pools, including soil organic matter. However, during 40 years of study of reforestation in the subtropical southeastern USA, repeated observations of above- and belowground carbon documented that significant gains in soil organic matter (SOM) in surface soils (0-7.5 cm) were offset by significant SOM losses in subsoils (35-60 cm). Here, we extended the observation period in this long-term experiment by an additional decade, and used soil fractionation and stable isotopes and radioisotopes to explore changes in soil organic carbon and soil nitrogen that accompanied nearly 50 years of loblolly pine secondary forest development. We observed that accumulations of mineral soil C and N from 0 to 7.5 cm were almost entirely due to accumulations of light-fraction SOM. Meanwhile, losses of soil C and N from mineral soils at 35 to 60 cm were from SOM associated with silt and clay-sized particles. Isotopic signatures showed relatively large accumulations of forest-derived carbon in surface soils, and little to no accumulation of forest-derived carbon in subsoils. We argue that the land use change from old field to secondary forest drove biogeochemical and hydrological changes throughout the soil profile that enhanced microbial activity and SOM decomposition in subsoils. However, when the pine stands aged and began to transition to mixed pines and hardwoods, demands on soil organic matter for nutrients to support aboveground growth eased due to pine mortality, and subsoil organic matter levels stabilized. This study emphasizes the importance of long-term experiments and deep measurements when characterizing soil C and N responses to land use change and the remarkable paucity of such long-term soil data deeper than 30 cm.

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