Divergent effects of legume cover crops on microbial necromass accumulation at surface versus subsurface soils in subtropical paddy fields

Area of Science:

• Agricultural Science
• Soil Science
• Microbiology

Background:

• Legume cover crops are vital for soil organic carbon (SOC) sequestration.
• Microbial necromass is crucial for SOC buildup, but its dynamics under cover crops in paddy soils, especially deep layers, are unclear.
• Amino sugars serve as biomarkers for microbial necromass.

Purpose of the Study:

• To investigate the impact of legume cover crops (milk vetch and hairy vetch) on microbial necromass accumulation at varying soil depths (0-60 cm) in paddy soils.
• To assess the contribution of microbial necromass to SOC sequestration under different management practices.
• To understand the role of nitrogen (N) availability in influencing microbial necromass dynamics.

Main Methods:

• A three-year field experiment was conducted using milk vetch (RM) and hairy vetch (RH) as cover crops.
• Microbial necromass was quantified using amino sugar biomarkers.
• Soil samples were analyzed at three depth increments: 0-20 cm, 20-40 cm, and 40-60 cm.

Main Results:

• Legume cover crops significantly increased microbial necromass in the surface layer (0-20 cm).
• This positive effect diminished with depth, showing a decrease in necromass in deeper layers (40-60 cm) compared to fallow controls.
• Cover crops with N fertilizer enhanced necromass accumulation in subsurface layers (20-60 cm) compared to unfertilized cover crops.

Conclusions:

• Legume cover crops have contrasting effects on microbial necromass, increasing it in surface soils but depleting it in deeper subsurface layers, potentially due to N limitation.
• Effective SOC sequestration strategies must consider microbial necromass dynamics across the entire soil profile.
• Optimizing cover crop management, possibly with N fertilization, is crucial for enhancing SOC sequestration in deeper soil horizons.