Changes in soil organic carbon, nitrogen, pH and bulk density with the development of larch (Larix gmelinii) plantations in China

Abstract

Under the government of China's environmental program known as Returning Farmland To Forests (RFTF), about 28 million hectares of farmland have been converted to tree plantation. This has led to a large accumulation of biomass carbon, but less is known about underground carbon-related processes. One permanent plot (25 years of observation) and four chronosequence plot series comprising 159 plots of larch (Larix gmelinii) plantations in northeastern China were studied. Both methods found significant soil organic carbon (SOC) accumulation (96.4 g C m À2 yr À1 ) and bulk density decrease (5.7 mg cm À3 yr À1 ) in the surface soil layer (0-20 cm), but no consistent changes in deeper layers, indicating that larch planting under the RFTF program can increase SOC storage and improve the physical properties of surface soil. Nitrogen depletion (4.1-4.3 g m À2 yr À1 ), soil acidification (0.007-0.022 pH units yr À1 ) and carbon/ nitrogen (C/N) ratio increase (0.16-0.46 per year) were observed in lessive soil, whereas no significant changes were found in typical dark-brown forest soil. This SOC accumulation rate (96.4 g m À2 yr À1 ) can take 39% of the total carbon sink capacity [net ecosystem exchange (NEE)] of larch forests in this region and the total soil carbon sequestration could be 87 Tg carbon within 20 years of plantation by approximating all larch plantations in northeastern China (4.5 Mha), showing the importance of soil carbon accumulation in the ecosystem carbon balance. By comparison with the rates of these processes in agricultural use, the RFTF program of reversing land use for agriculture will rehabilitate SOC, soil fertility and bulk density slowly (o69% of the depletion rate in agricultural use), so that a much longer duration is needed to rehabilitate the underground function of soil via the RFTF program. Global forest plantations on abandoned farmland or function to protecting farmland are of steady growth and our findings may be important for understanding their underground carbon processes.

Glomalin contributed more to carbon, nutrients in deeper soils, and differently associated with climates and soil properties in vertical profiles

“…Until recently, the properties and dynamics of C and nutrients in the deep soils were largely ignored 38 . Forest growth could result in SOC accumulation, taking a large percentage of whole forest ecosystem carbon sequestration 34,50 ; Moreover, deep soils showed a contrary tendency than did in the surface soils in nutrient dynamics 34,50 . Our findings indicate that the climatic influences on glomalin accumulation can reach as deep as 100 cm in soil depth, and possible underlying reason should be related with soil-mycorrhizal interactions.…”

“…In China, the grain production in Songnen Plain accounts for over 10% of total commercial grain output in China 54 , owing to fertile black soils in this region 55,56 . However, soil degradation restricts agricultural development 57,58 as shown be a 50% reduction in SOM and soil fertility 59 , serious saline-alkalinization 60 and soil physical degradation of soil bulk density increase and decrease of total porosity 50 . The following suggestions for northeastern China farmland development could be proposed based on the results of this study.…”

“…The 5 soil depths were respectively 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm and 80–100 cm. All roots and gravel were carefully removed, the soil samples were ground, and passed through a 0.25-mm sieve prior to laboratory analysis 50 .…”

“…Soil gravimetric water was calculated as follows: [(Fresh weight − Air-dried weight) / Dry weight] × 100%. Soil bulk density was calculated as follows: Air-dried soil mass/soil volume of 400 cm 3 50 .…”

SOM and Biomass C Stocks in Degraded and Undisturbed Andean and Coastal Nothofagus Forests of Southwestern South America

“…New measurements of soil bulk density should be performed only in the exclusion plots in order to test this hypothesis. With respect to UNF at both locations, the low soil bulk density at 0-40 cm depth can probably be explained by the larger amount of roots found in this condition which, according to [42] who studied their effect on several soil parameters in temperate woodlands, contribute to lower the bulk density by action of root penetration across the soil profile.…”