Farming Influence on Physical-Mechanical Properties and Microstructural Characteristics of Backfilled Loess Farmland in Yan’an, China

Lina Ma,Songfeng Guo,Qiangbing Huang,Shengwen Qi,Xinbao Yu,Bowen Zheng

doi:10.3390/su12145516

Abstract

A gigantic project named Gully Land Consolidation (GLC) was launched in the hill-gully region of the Chinese Loess Plateau in 2011 to cope with land degradation and create new farmlands for cultivation. However, as a particular kind of remolded loess, the newly created and backfilled farmland may bring new engineering and environmental problems because the soil structure was disturbed and destroyed. In this study, current situations and characteristics of GLC are introduced. Test results show that physical-mechanical properties and microstructural characteristics of backfilled loess of one-year and five-year farmland are significantly affected by the Gully Land Consolidation project. Compared to natural loess, the moisture content, density, and internal friction angle of backfilled loess increase. On the contrary, the porosity, plasticity index, particle size index, and cohesion index decrease. Through SEM tests, it is observed that the particles of backfilled loess are rounded, with large pores filled with crushed fine particles, which results in skeleton strength weakness among particles and pores. The pore size distribution (PSD) of the four types of loess (Q3 loess, Q2 loess, one-year farmland, and five-year farmland) was measured using mercury intrusion porosimetry (MIP) tests, showing that the pore size of Q3 loess is mainly mesopores 4000–20,000 nm in size, accounting for 67.5%. The Q2, five-year, and one-year farmland loess have mainly small pores 100–4000 nm in size, accounting for 52.5%, 51.7%, and 71.7%, respectively. The microscopic analysis shows that backfill action degrades the macropores and mesopores into small pores and micropores, leading to weak connection strength among soil particles, which further affects the physical-mechanical properties of loess. The disturbance of backfilled loess leads to an obvious decrease in cohesion and a slight increase in internal friction compared to natural loess. The farming effect becomes prominent with increased backfill time, while the loess soil moisture content increases gradually. Both the cohesion and internal friction of the backfilled loess soil decrease to different degrees. This study is helpful to investigate sustainable land use in the Chinese Loess Plateau and similar areas.

Highlights

The Chinese Loess Plateau (CLP) covers an area of 640,000 km2 in the upper and middle reaches of the Yellow River
Test results show that physical-mechanical properties and microstructural characteristics of backfilled loess of one-year and five-year farmland are significantly affected by Gully Land Consolidation
With the development of filling time, the physical indices of backfilled loess tend to resemble those of natural loess, and the fluctuation amplitudes gradually decrease

Summary

Introduction

The Chinese Loess Plateau (CLP) covers an area of 640,000 km in the upper and middle reaches of the Yellow River. It has multiple crisscrossing hills and gullies with fractured geological structures [1]. Since the Chinese government implemented the GFG project in 1999 to convert farmland to forests, shrubland, and grassland, the vegetation coverage rate of the CLP increased significantly, from 31.6% in 1999 to about 65% in 2017 [8], thereby reducing soil erosion and water loss to a certain extent [9,10]. In Yan’an, the total area of arable land has reduced by more than half, creating an urgent need for new farmland and a shortage in grain production, which are the main obstacles in sustainable agricultural development [12,13]

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Results

Conclusion