Greenhouse gas emissions during alfalfa cultivation: How do soil management and crop fertilisation of preceding maize impact emissions?

Context

The use of alfalfa in rotation with intensive crops is common practice to mitigate the physical and chemical issues arising from intensive farming practices. However, there is a dearth of studies on this practice. Given the current concern regarding climate change and the significant impact agriculture has on greenhouse gas (GHG) emissions, understanding the emissions associated with this practice, as well as the most suitable soil and crop management techniques for their mitigation, is of paramount importance.

Objective

The present study aimed to (i) quantify emissions of N₂O, CO₂ and CH₄ in an alfalfa crop following a maize cropping scenario; (ii) to determine which tillage system generates the lowest GHG emissions, and; (iii) to determine how N fertilisation from a preceding intensive maize crop affects GHG emissions during alfalfa cropping period.

Methods

A three-year field experiment (2019, 2020 and 2021) was conducted to assess the emissions of N₂O, CO₂ and CH₄ from alfalfa cultivation following a three-year period of irrigated maize. Two soil management practices (no-tillage and conventional tillage) were implemented during both the maize cropping period and the alfalfa establishment. Additionally, the nitrogen (N) fertilisation rates applied to the preceding maize crop were included as a treatment (0, 200, and 400 kg N ha⁻¹, corresponding to zero, medium, and high fertilisation levels, respectively) in a randomized block design with two factors.

Results

Emissions of N₂O in alfalfa ranged from 0.05 to 0.32 mg N₂O-N m⁻² day⁻¹, being significantly higher only during first month of sampling in the treatments that had received fertilisation. CO₂ emissions ranged from 1158 to 4258 mg CO₂-C m⁻² day⁻¹. Year-average CH₄ fluxes were −0.27 g C ha⁻¹ day⁻¹. The average total dry matter produced by alfalfa was 17700 kg ha⁻¹ year⁻¹, being higher for the no-tillage treatment, though significantly so only during first month of sampling.

Conclusions

Under Mediterranean conditions, the tillage system and mineral N fertilizer rates have a relative effect on greenhouse gas emissions during the alfalfa cropping period. Plots without N fertilization initially produced lower N₂O emissions and higher total dry matter, resulting in the lowest scaled emissions. For the tillage treatment, no significant differences were found in emission dynamics, which may be due to the fact that alfalfa does not involve soil disturbance, leading to a homogenization of the treatments. However, the NT treatment showed lower scaled emissions due to higher yields in the first year. Therefore, alfalfa cultivation is characterized by low GHG emissions, high yields, and a notable capacity to mitigate the negative effects of previous intensive crops.

Significance

This study provides data on GHG emissions during alfalfa cropping in the typical Maize-alfalfa crop rotation under Mediterranean irrigated systems, which is useful for new agricultural policies aimed at reducing pollution in the agricultural sector. Additionally, it demonstrates the capability of this crop to mitigate adverse agronomic and environmental conditions caused by preceding intensive farming practices.