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Reforestation of agricultural land with mixed-species environmental plantings (native trees and shrubs) can contribute to mitigation of climate change through sequestration of carbon. Although soil carbon sequestration following reforestation has been investigated at site- and regional-scales, there are few studies across regions where the impact of a broad range of site conditions and management practices can be assessed. We collated new and existing data on soil organic carbon (SOC, 0–30 cm depth, N = 117 sites) and litter ( N = 106 sites) under mixed-species plantings and an agricultural pair or baseline across southern and eastern Australia. Sites covered a range of previous land uses, initial SOC stocks, climatic conditions and management types. Differences in total SOC stocks following reforestation were significant at 52% of sites, with a mean rate of increase of 0.57 ± 0.06 Mg C ha −1 y −1 . Increases were largely in the particulate fraction, which increased significantly at 46% of sites compared with increases at 27% of sites for the humus fraction. Although relative increase was highest in the particulate fraction, the humus fraction was the largest proportion of total SOC and so absolute differences in both fractions were similar. Accumulation rates of carbon in litter were 0.39 ± 0.02 Mg C ha −1 y −1 , increasing the total (soil + litter) annual rate of carbon sequestration by 68%. Previously-cropped sites accumulated more SOC than previously-grazed sites. The explained variance differed widely among empirical models of differences in SOC stocks following reforestation according to SOC fraction and depth for previously-grazed ( R 2 = 0.18–0.51) and previously-cropped ( R 2 = 0.14–0.60) sites. For previously-grazed sites, differences in SOC following reforestation were negatively related to total SOC in the pasture. By comparison, for previously-cropped sites, differences in SOC were positively related to mean annual rainfall. This improved broad-scale understanding of the magnitude and predictors of changes in stocks of soil and litter C following reforestation is valuable for the development of policy on carbon markets and the establishment of future mixed-species environmental plantings. [ABSTRACT FROM AUTHOR]

In extensively cleared areas of temperate Australia, large-scale mixed-species environmental revegetation is often recommended to address land and water degradation in rural landscapes. Although survival and growth are central to the success of revegetation, there is very limited information on this aspect of practice. We investigated how site and management practices influence early growth by surveying 29 planted or direct-seeded sites, covering ages from 3-6 years, in north central Victoria, Australia. We measured heights and crown widths of trees and shrubs at each site and estimated average growth over time at the scale of: (i) individual trees or shrubs (mean height increment) and (ii) stand (vegetation cover increment and a 'height integral' increment variable, which integrates differences in both height and stand density). Relationships between growth increment variables and site factors, including climate, site preparation and land-use history, were investigated to identify the main factors influencing early growth rates. Mean annual increment in height of individual trees was significantly higher for trees established from tubestock than those from direct seeding. Multiple linear regression models for two growth variables, cover increment and height integral increment, explained 61% and 38% of the variation, respectively, but with high uncertainty. Ploughing, pre-establishment weed control and fertiliser addition improved growth rates. Revegetation method had a significant effect on early growth; at the scale of individuals, growth rates were higher for planted tubestock than for direct seeding, while at the stand scale, growth rates were higher for direct seeding than for planted tubestock. [ABSTRACT FROM AUTHOR]

Reforestation of agricultural lands with mixed-species environmental plantings can effectively sequester C. While accurate and efficient methods for predicting soil organic C content and composition have recently been developed for soils under agricultural land uses, such methods under forested land uses are currently lacking. This study aimed to develop a method using infrared spectroscopy for accurately predicting total organic C (TOC) and its fractions (particulate, POC; humus, HOC; and resistant, ROC organic C) in soils under environmental plantings. Soils were collected from 117 paired agricultural-reforestation sites across Australia. TOC fractions were determined in a subset of 38 reforested soils using physical fractionation by automated wet-sieving and 13 C nuclear magnetic resonance (NMR) spectroscopy. Mid- and near-infrared spectra (MNIRS, 6000–450 cm −1 ) were acquired from finely-ground soils from environmental plantings and agricultural land. Satisfactory prediction models based on MNIRS and partial least squares regression (PLSR) were developed for TOC and its fractions. Leave-one-out cross-validations of MNIRS-PLSR models indicated accurate predictions ( R 2 > 0.90, negligible bias, ratio of performance to deviation > 3) and fraction-specific functional group contributions to beta coefficients in the models. TOC and its fractions were predicted using the cross-validated models and soil spectra for 3109 reforested and agricultural soils. The reliability of predictions determined using k -nearest neighbour score distance indicated that >80% of predictions were within the satisfactory inlier limit. The study demonstrated the utility of infrared spectroscopy (MNIRS-PLSR) to rapidly and economically determine TOC and its fractions and thereby accurately describe the effects of land use change such as reforestation on agricultural soils. [ABSTRACT FROM AUTHOR]