Forest restoration in China’s Loess Plateau: Study explores the soil structure and carbon content of the arid plateau’s tree plantations

Soil organic carbon (SOC) plays a significant role in the global carbon cycle as a regulator of climate change. Improving SOC stocks is a major political priority worldwide and is a key consideration in policy initiatives such as the Soil Strategy for 2030 and the Nature Restoration Regulation, which has binding targets to improve the condition of forested land.

A China-based study reveals that broadleaf forest shows better SOC accumulation, more stable soil structure, and higher resistance to erosion in semi-arid land than mixed or coniferous forests. The work, based on findings in the Loess Plateau, also highlights the contribution of different soil types to aggregate formation, with significant implications for forestry-based ecological restoration and climate mitigation strategies.

The researchers carried out extensive sampling across three altitudinal gradients (from 940-1240 m) in the study region, which has seen significant planting of trees since the late 1990s. The forested area comprises approximately 80% broadleaf species, dominated by sawtooth oak (Quercus acutissima), and 20% conifers, dominated by Chinese red pine (Pinus tabuliformis). Some forest stands are mixed (broadleaf and conifer) and others exclusively broadleaf or coniferous.

The study used sample plots of 20 m² in size across the three forest types, all with similar slopes and tree distribution. The researchers took soil samples from the surface layer to one metre depth. They then used advanced statistical methods to analyse soil physical and chemical properties, soil aggregate stability, and how aggregates of different particle sizes contributed to SOC.

Soil aggregates are porous structural bodies of different sizes formed by mineral particles and organic-inorganic complexes through rearrangement, flocculation (clumping), and cementation. They constitute the basic material of soil structure, and their composition plays an important role in soil anti-erosion capacity and soil quality improvement. Previous studies have shown that soil aggregates are the main place where organic carbon exists, and their physical protection of organic carbon is the main mechanism of soil carbon fixation.

The researchers found that broadleaf forest exhibited better soil carbon storage than either mixed or coniferous forest. Mixed forests stored the second most, and coniferous forests the least. The same ranking of forest types was also apparent for soil aggregate stability when using the indicators of mean weight diameter and geometric mean diameter. This suggested that broadleaf forests have better potential to resist erosion, which can undermine the SOC stored in topsoil.

Across the forest types, enriched SOC was generally found in the top 0-20 cm of soil. Aggregates with larger soil particles (those of over 2 mm in size) were most common in all forest types, representing about 40% of all soil aggregates overall. These so-called ‘macroaggregates’ contributed significantly more – up to 15 times more – to SOC than aggregates comprising smaller particles (0.25–0.5 mm).

There were also differences in SOC and soil aggregates at various altitudes. As altitude increased, SOC in coniferous forest decreased at first in all soil layers and then increased; in broadleaf forests the researchers observed the same decrease-increase pattern in the top 60 cm but with a gradual increase in the 60–100 cm layer, while in mixed forests there was a gradual increase in SOC across all layers. A similar pattern emerged for soil aggregates: macroaggregates decreased and then increased with altitude in coniferous and broadleaf forests but showed a gradual increase in mixed forests.

The study’s rigorous, wide-scale analysis provides valuable insight into how different forest structures influence SOC and aggregation, also offering novel insights into carbon cycling across soil depths and aggregate sizes. Despite the regional context of the study, the findings have broader application to international climate agreements, which underlines the importance of nature-based solutions.

Europe’s forests contain a large proportion of conifers, and much afforestation has been carried out with coniferous species. However, there have been moves to create more diverse mixed forests in parts of Europe in recent decades. Meanwhile, the EU’s semi-arid regions are concentrated in southern Europe, and include parts of Portugal, Spain, Italy, Greece and the Balkans. These are regions with lower levels of coniferous forest cover than central and northern Europe. Forestry work under the Nature Restoration Regulation in these regions could well take note of the findings of the study, as well as other research which has advocated restoration focused on broadleaved trees.

Reference: 

Zhao, Q., Xu, D., Qian, J., Yan, Y., Guo, F., Liu, W. and Liu, Y., 2025. Effects of Forest Restoration Structure on Soil Aggregate‐Associated Organic Carbon in the Loess Plateau. Land Degradation & Development.  https://doi.org/10.1002/ldr.70044

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“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.