Hidden miners – the roles of cover crops and soil microorganisms in phosphorus cycling through agroecosystems

Abstract

Background Phosphorus (P) is a limiting nutrient in many agroecosystems and costly fertilizer inputs can cause negative environmental impacts. Cover crops constitute a promising management option for sustainable intensification of agriculture. However, their interactions with the soil microbial community, which is a key driver of P cycling, and their effects on the following crop, have not yet been systematically assessed. Scope We conducted a meta-analysis of published field studies on cover crops and P cycling, focusing on plantmicrobe interactions. Conclusions We describe several distinct, simultaneous mechanisms of P benefits for the main crop. Decomposition dynamics, governed by P concentration, are critical for the transfer of P from cover crop residues to the main crop. Cover crops may enhance the soil microbial community by providing a legacy of increased mycorrhizal abundance, microbial biomass P, and phosphatase activity. Cover crops are generally most effective in systems low in available P, and may access 'unavailable' P pools. However, their effects on P availability are difficult to detect by standard soil P tests, except for increases after the use of Lupinus sp. Agricultural management (i.e. cover crop species selection, tillage, fertilization) can improve cover crop effects. In summary, cover cropping has the potential to tighten nutrient cycling in agricultural systems under different conditions, increasing crop P nutrition and yield.

Options for Improved Phosphorus Cycling and Use in Agriculture at the Field and Regional Scales

“…The differing capabilities of agricultural crops to mobilize sparingly soluble P have generated interest in the ability of certain crops to provide P benefits to subsequent or concurrent crops. The major pathways by which P‐mobilizing crops may benefit subsequent crops include P accumulation in crop tissues followed by mineralization to plant‐available Pi, enhancement of soil microbial biomass and activity (especially AMF), and rhizosphere modification (through exudation of carboxylates, enzymes, and/or protons that mobilize recalcitrant Pi and Po) (Hallama et al, 2018). However, Nuruzzaman et al (2005) noted that rhizosphere modification is thought to play a limited role in provision of P benefits to subsequent crops due to the relatively brief duration of rhizosphere modification.…”

“…As Menezes‐Blackburn et al (2018) discussed, plants with efficient P uptake mechanisms should be useful for mobilizing recalcitrant legacy P and helping make it available to the subsequent crop. Phosphorus mobilization by cover crops appears to result in greater relative yield increases in the subsequent crop when available soil Pi is low and where traits affecting P mobilization should be fully expressed; however, these benefits may be limited if total soil P is low (Hallama et al, 2018). Addition of P fertilizers to low‐P soils can enhance P uptake by P‐mobilizing crops, potentially increasing the P benefit to a following crop (Cavigelli and Thien, 2003; Arcand et al, 2010), suggesting the need to further explore adding soil P amendments at strategic points in the crop rotation.…”

“…provided greater benefits to subsequent wheat, relative to canola ( Brassica napus L.) or cereal rye ( Secale cereale L.) (Doolette et al, 2019). Results of a meta‐analysis indicate that the Fabaceae family of legumes shows great potential for benefits to subsequent crops, although only a few individual species from this family increased P supply to subsequent crops, relative to control treatments (Hallama et al, 2018). While Lupinus spp.…”

“…While Lupinus spp. show great potential for P mobilization for their own use, their benefits to subsequent crops are generally lower than expected, perhaps due to these species’ relatively low biomass production (Hallama et al, 2018). …”

“…Harnessing P mobilization processes used by crops for increased PUE in agricultural systems is promising; however, the results to date have been inconsistent and the need to further identify and understand the mechanisms contributing to any observed benefits is apparent. In crop sequences, decomposition dynamics and their impact on P release from crop residues are of particular interest, as are the relative roles of shoot versus root residues (Kamh et al, 1999; Nuruzzaman et al, 2005; Hallama et al, 2018). In intercrops, the role of crop rooting patterns (i.e., root avoidance vs. intermingling) in creating complementary versus facilitative interactions among crop species requires further investigation, especially under field conditions where roots are less restricted relative to pot studies (Darch et al, 2018).…”