Net aboveground production (4.1—9.5 Mg°ha—1) across a series of edaphic climax forests was highly correlated with field measurements of soil N mineralization (26—84 kg°ha—1°yr—1; r2 = 0.902, P < .001) and with soil silt + clay content (5—74%; r2 = 0.883, P < .001). Soil N mineralization was positively correlated with litter production and N and P return in litter. Soil N mineralization was negatively correlated with litter C:N and C:P ratios and with efficiency of P use in litter production. Efficiency of N use in litter production declined with increasing N mineralization except for inefficient use of N in a hemlock stand at low N mineralization. Changes of litter quality across the mineralization—soil texture gradient were due to species replacement rather than changes in litter quality within each species. Nitrification was not correlated with aboveground production. Both mineralization and nitrification were highly correlated with humus P content. Differences in nitrification among the soils appeared to be related to PO4—P supply in the spring and early summer and to NH4—N supply in mid— to late summer.

Abstract. The purpose of this study was to relate regional variation in litter mass-loss rates (first year) in pine forests to climate across a large, continental-scale area. The variation in mass-loss rate was analyzed using 39 experimental sites spanning climatic regions from the subarctic to subtropical and Mediterranean: the latitudinal gradient ranged from 31 N to 70 N and may represent the the largest geographical area that has ever been sampled and observed for the purpose of studying biogeochemical processes. Because of unified site design and uniform laboratory procedures, data from all sites were directly comparable and permitted a determination of the relative influence of climate versus substrate quality viewed from the perspective of broad regional scales.Simple correlation applied to the entire data set indicated that annual actual evapotranspiration (AET) should be the leading climatic constraint on mass-loss rates (R2d = 128 0.496). The combination of AET, average July temp. and average annual temp. could explain about 70% of the sites' variability on litter mass-loss. In an analysis of 23 Scots pine sites north of the Alps and Carpatians AET alone could account for about 65% of the variation and the addition of a substrate-quality variable was sufficiently significant to be used in a model.The influence of litter quality was introduced into a model, using data from 11 sites at which litter of different quality had been incubated. These sites are found in Germany, the Netherlands, Sweden and Finland. At any one site most (> 90%/6) of the variation in mass-loss rates could be explained by one of the litter-quality variables giving concentration of nitrogen, phosphorus or water solubles. However, even when these models included nitrogen or phosphorus even small changes in potential evapotranspiration resulted in large changes in early-phase decay rates.Further regional subdivision of the data set, resulted in a range of strength in the relationship between loss rate and climatic variables, from very weak in Central Europe to strong for the Scandinavian and Atlantic coast sites (Rdj = 0.912; AET versus litter mass loss). Much of the variation in observed loss rates could be related to continental versus marine/Atlantic influences. Inland locations had mass-loss rates lower than should be expected on the basis of for example AET alone. Attempts to include seasonality variables were not successful. It is clear that either unknown errors and biases, or, unknown variables are causing these regional differences in response to climatic variables. Nevertheless these results show the powerful influence of climate as a control of the broad-scale geography of mass-loss rates and substrate quality at the stand level.Some of these relationships between mass-loss rate and climatic variables are among the highest ever reported, probably because of the care taken to select uniform sites and experimental methods. This suggest that superior, base line maps of predicted mass-loss rates could be produced using climatic...

Standing crop, rates of production, mortality, decomposition, and nitrogen dynamics of two size classes of fine roots (0—05 mm and 0.5—3.0 mm diameter) were estimated for 1 yr in a 53—yr—old red pine (Pinus resinosa Ait.) plantation and in an adjacent 80—yr—old mixed hardwood stand in north—central Massachusetts. Dry matter of live fine roots was higher in the hardwoods (mean = 6.1 Mg/ha; annual range 3.6—8.6 Mg/ha) than in the plantation (mean = 5.1 Mg/ha; annual range 2.5—7.8 Mg/ha.) Dead root mass was similar in the hardwoods (mean = 4.4 Mg/ha) and the plantation (mean = 4.0 Mg/ha). Nitrogen standing crop of live roots in the hardwoods was higher than in the plantation (mean = 65 kg/ha and 42 kg/ha, respectively). Net fine root production was estimated from changes in standing crop. Production estimates ranged from 4.1 to 11.4 Mg°ha—1°yr—1 in the hardwoods and from 3.2 to 10.9 Mg°ha—1°yr—1 in the plantation, depending on the assumptions made in the calculations. Concurrent estimates of total nitrogen requirement for this production ranged from 73 to 184 kg°ha—1°yr—1 in the hardwoods and from 44 to 122 kg°ha—1°yr—1 in the plantation. Decomposition, measured as mass loss from buried cloth bags, was °20% in 0.4—mm mesh bags and as high as 47% in 3—mm mesh bags after 1 yr. Integrating production and nitrogen requirements with estimates of decomposition rates and nitrogen mineralization for these ecosystems suggested that the lower estimates of production are more accurate.

Long-term decomposition data are presented for several types of foliar and fine root litter in different stands in Wisconsin and Massachusetts, U.S.A. Changes in mass remaining as well as nitrogen and carbon fraction (extractives, cellulose, lignin) concentration are reported. Three models were developed for describing change in mass remaining with time: a litter-specific exponential decay function (statistical fit of data for each litter type), a generalized exponential decay function (k predicted from initial litter chemistry), and a carbon fraction model that calculates the weight loss of each carbon fraction individually as a function of current carbon chemistry regardless of litter type. The exponential decay function fits all litter data well for the portion of decomposition described here, but would not be appropriate for modeling longer term decomposition. Both the generalized and carbon fraction models predicted weight loss accurately. All litter types had similar carbon fraction chemistries at the end of the first phase of decomposition described here and also exhibited a narrow range of changes in nitrogen concentration per unit weight loss. It is concluded that the length of time required to convert litter into soil organic matter and the chemistry of the material produced by this process can be predicted from initial litter chemistry and (or) relatively short-term litter decay data. Key words: immobilization, mineralization, humus, lignin, cellulose, extractives.