Evaluating foliar characteristics as early indicators of plant response to biochar amendments

Abstract

Biochar amendments are widely promoted for increasing soil nutrient availability and plant yield, and although many plant species show positive growth responses, neutral to negative effects can also occur. Although biochar effects on plant yield may not yet be evident in the first growing season after amendment, foliar characteristics such as nitrogen (N) and chlorophyll concentrations are known to respond to changes in soil fertility and therefore may provide an early indication of plant performance. In this study, we investigated the effects of contrasting biochar treatments on soil nitrogen (N) forms and plant chlorophyll, and the relationships between soil and plant nitrogen status, for two economically important conifer species, Abies balsamea (L.) Mill and Picea pungens Englem., as well as forb and grass weeds. We applied two types of pyrolysis biochars (hereafter, USB and BGR) that differed in feedstock type (pine wood versus mixed forest harvesting residue) at contrasting application rates (low, high, and untreated control) to a fine sandy loam soil. We measured soil water content (SWC), pH, and inorganic nitrogen (NH4+, NO3−, and total inorganic N (TIN)), as well as plant foliar N, and chlorophyll content (Chl-a, -b, -ab), and fluorescence (Fv/Fm). SWC response was variable whereas all treatments except USB low increased pH. All treatments decreased inorganic N concentrations relative to the control, although differences diminished over the study period. USB high decreased Picea foliar N concentration whereas all treatments decreased grass foliar N. BGR decreased Abies Fv/Fm (low) and Picea Chl-a, Chl-ab (both levels), and Chl-b (low), whereas USB increased Abies Chl-ab (high). There was a weak positive relationship between foliar N and soil NH4+ (r2 = 0.14) and NO3− (r2 = 0.11) in Abies seedlings, and between foliar N and TIN (r2 = 0.16, 0.11) in both conifers. Abies foliar N showed a weak positive effect on Chl-a (r2 = 0.22), Chl-b (r2 = 0.10), and Chl-ab (r2 = 0.22). Soil NO3− (r2 = 0.14) and TIN (r2 = 0.14) showed a weak negative correlation with Picea foliar Fv/Fm. This study shows that biochar type and amendment level influence soil and plant nitrogen responses, as well as their relationships, and responses may differ between plant species. Evaluating foliar characteristics shows promise as an early indicator of plant response to biochar, and may provide additional important information for evaluating contrasting biochars as potential sustainable soil amendments.