Independence of seasonal patterns of root functional traits and rooting strategy of a grass‐clover sward from sward age and slurry application

Abstract

Root functional traits (RFTs) are crucial for understanding nutrient cycling processes. However, seasonal variation of RFTs and their potential changes in root foraging pattern have been almost ignored in field studies. In this work, belowground net primary production (BNPP, g m−2), root length density (RLD, km m−2), specific root length (SRL, m g−1), root N concentration and C/N ratio in defoliated grass‐clover swards of different sward ages and receiving different cattle slurry levels were investigated for 2 years. All measured RFTs showed significant seasonal variation: in general, seasonal variation was most marked between primary growth period (until heading stage of perennial ryegrass) and subsequent regrowth periods following defoliation; meanwhile, the rooting pattern also differed. These phenomena were not affected by current management or sward composition, but were correlated with plant C availability. Daily BNPP and RLD rates varied 7‐ to 8‐fold in the growing season. Moreover, BNPP and RLD were asynchronous due to seasonal variation in SRL. Rooting pattern and the relationship with shoot phenology are discussed. We conclude that root functional traits vary with season; thus, roots which develop in different seasons may be involved in belowground nutrient cycling processes with different turnover characteristics; trade‐off among traits may also involve temporal variation. Root‐C related investigations in grass‐clover swards should include root mass sampling rather than use of root length observations alone.

Root growth and turnover in perennial forages as affected by management systems and soil depth

“…The observed slower root elongation in the regrowth periods following the first defoliation is consistent with a previous observation in grass-clover swards (Chen et al 2016), which was explained by a lower watersoluble carbohydrate availability for root growth in the regrowth periods following defoliation, compared to the primary growth. However, higher root growth rates during summer than spring were reported from a study conducted with tall fescue in France (Bélanger et al 1994), which is contrary to our observations.…”

“…Seasonal variations in root length and diameter of the grass-only management systems (SLU and CAN) over the 60-cm soil profile are consistent with those reported under similar conditions in shallower soil layers (0-30 to 0-40 cm; Beyrouty et al 1990;Chen et al 2016), suggesting that deep root development (30-60 cm) also shows a seasonal pattern. The observed patterns in root length over two growing seasons is also similar to those observed for root biomass by Steen (1989).…”

“…Root traits exhibit strong seasonal patterns (Chen et al 2016), which are likely to be modulated by soil depth and management practices (Fan et al 2016;Lazicki et al 2016) and, hence, to influence rootderived soil C fluxes (Poirier et al 2018). Defoliation in perennial grass pasture species was shown to increase root turnover, while also inducing a smaller root system (McInenly et al 2010;Reid et al 2015).…”

Nitrous oxide emissions from grass–clover swards as influenced by sward age and biological nitrogen fixation

“…Reasons for the generally observed low emissions from legume‐based swards might include a combination of high N uptake by plants due to the high root length and root density commonly observed in grass–legume swards (Chen et al., 2016), together with the low levels of nitrate in the root zone. For instance, soil N concentrations rarely exceeded 20 mg NO 3 ‐N/l , which has been indicated as the critical value for increased N 2 O fluxes at high water‐filled pore space values during laboratory experiments (Senbayram, Chen, Budai, Bakken, & Dittert, 2012).…”

Responses of Fine Root Functional Traits to Soil Nutrient Limitations in a Karst Ecosystem of Southwest China

“…Season is another important factor that influences fine root functional traits as a result of differing light intensities, soil temperature, water, microbial abundances, and enzymatic activities [36,37,63,64]. Firstly, higher light and temperature conditions can improve the photosynthetic capacity and further increase plant growth in the summer season compared to the winter season [65].…”