Agroecosystem diversification with legumes or non-legumes improves differently soil fertility according to soil type

Plant diversification through crop rotation or agroforestry is a promising
way to improve sustainability of agroecosystems. Nonetheless, criteria to
select the most suitable plant communities for agroecosystems
diversification facing contrasting environmental constraints need to be
refined. Here, we compared the impacts of 24 different plant communities
on soil fertility across six tropical agroecosystems: either on highly
weathered Ferralsols, with strong P limitation, or on partially weathered
soils derived from volcanic material, with major N limitation. In each
agroecosystem, we tested several plant communities for diversification, as
compared to a matching low diversity management for their cropping system.
Plant residue restitution, N, P and lignin contents were measured for each
plant community. In parallel, the soil under each community was analyzed
for organic C and N, inorganic N, Olsen P, soil pH and nematode community
composition. Soil potential fertility was assessed with plant bioassays
under greenhouse controlled climatic conditions. Overall, plant
diversification had a positive effect on soil fertility across all sites,
with contrasting effects depending on soil type and legumes presence in
the community. Communities with legumes improved soil fertility indicators
of volcanic soils, which was demonstrated through significantly higher
plant biomass production in the bioassays (+18%) and soil inorganic N
(+26%) compared to the low diversity management. Contrastingly,
communities without legumes were the most beneficial in Ferralsols, with
increases in plant biomass production in the bioassays (+39%), soil Olsen
P (+46%), soil C (+26%), and pH (+5%). Piecewise structural equation
models with Shipley’s test revealed that plant diversification impacts on
volcanic soil fertility were related to soil N availability, driven by
litter N. Meanwhile, Ferralsols fertility was related to soil P
availability, driven by litter P. These findings underline the importance
of multifactorial and multi-sites experiments to inform trait-based
frameworks used in designing optimal plant diversification in
agroecological systems.