Hey Glen,
This is a fantastic question! Recommendations for green manure/cover crop (GMCCs) management differ based on your climate and season. Most recommendations for tropical and subtropical agroecosystems are to leave cover crop residues on the surface, to slow decomposition and cover the soil, thereby cooling the soil and retaining soil moisture as well as Nitrogen (e.g., Vieira et al., 2009; Erenstein, 2002). This is the opposite in temperate climates! What’s the reasoning behind this? It is multifaceted but has a lot to do with losses from the system. I did my master’s research on this topic, so I apologize for the following rant.
Nitrogen and carbon decomposition occur faster when plant residues are incorporated than left on the soil surface in tropical conditions (Lynch et al., 2016; Jani et al., 2019; Mulvaney et al., 2017; Mulvaney et al., 2010; Abera et al., 2014). In warm, wet climates microbial breakdown and leaching of nutrients is rapid. When residues are incorporated, the increased access to residues increases the microbial breakdown of N and C (among other things). This also heats up and areates the upper horizons of the soil. These increases in losses due to higher moisture and temperature outweigh the losses that would be encountered solely by N volatilization caused by drying in the sun on the soil surface. The thicker the cover crop, the better. The top layer of residue helps protect the lower layers as well as the soil. The difference between practices would be less if the cover crop or residues are not thick.
What exactly is the numerical difference between incorporating and not incorporating is a much more detailed question that depends on your specific season, let alone climate. The wetter and hotter your season of residue decomposition is, the better it is to leave residues on the soil surface rather than incorporate them. The cooler, drier your season after incorporation is, the better it is to incorporate to protect from N volatilization. There are studies that vary across the literature showing different numbers on N and C losses given both practices in unique agroecosystems, but general guidelines are to incorporate in cool climates and leave residues on the soil surface in hot climates.
As far as teaching this, below is a screenshot of the Slide I show ECHO interns when talking about this topic. When I worked in East Africa with some farmers, I took samples from farms that were no-till vs. tilled and asked farmers which one they would rather have on their farm and why and then shared what the farmer’s practices were. I’m happy to share more if you need!
References:
- Abera, G., Wolde-Meskel, E., Bakken, L. 2014. Unexpected high decomposition of legume residues in dry season soils from tropical coffee plantations and crop lands. Agronomy for Sustainable Development 34 (3):667-676.
- Erenstein, O. 2002. Crop residue mulching in tropical and semi-tropical countries: An evaluation of residue availability and other technological implications. Soil and Tillage Research 67:115-133.
- Jani, A.D., Mulvaney, M.J., Enloe, H.A., Erickson, J.E., Leon, R.G., Rowland, D.L., and Wood, C.W. 2019. Peanut residue distribution gradients and tillage practices determine patters of nitrogen mineralization. Nutrient Cycling in Agroecosystems 113:63-76.
- Lynch, M.J., Mulvaney, M.J., Hodges, S.C., Thompson, T.L., and Thomason, W.E. 2016. Decomposition, nitrogen and carbon mineralization from food and cover crop residues in the central plateau of Haiti. SpringerPlus 5(1):973–981.
- Mulvaney, M.J., Balkcom, K.S., Wood, C.W., and Jordan, D. 2017. Peanut residue carbon and nitrogen mineralization under simulated conventional and conservation tillage. Agronomy Journal 109:696-705.
- Mulvaney, M.J., Wood, C.W., Balkcom, K.S., Shannon, D.A., and Kemble, J.M. 2010. Carbon and nitrogen mineralization and persistence of organic residues under conservation and conventional tillage. Agronomy Journal 102(5):1425–1433.
- Vieira, F.C.B., Bayer, C., Zanatta, J.A., Mielniczuk, J., and Six, J. 2009. Building up organic matter in a subtropical pleudult under legume cover-crop-based rotations. Soil Science Society of America Journal. September-October:1699-1706.