Ecological Enzyme Stoichiometry Reveals Seasonal and Treatment-Induced Constraints on C, N, and P Dynamics in Olive Orchard Soils

Ecological Enzyme Stoichiometry Reveals Seasonal and Treatment-Induced Constraints on C, N, and P Dynamics in Olive Orchard Soils

Olive orchards in semi-arid Mediterranean regions face critical challenges including soil degradation, drought, and erosion, threatening their long-term sustainability. Seasonal monitoring can provide critical insights into soil health dynamics and assess the effect of nature-based solutions (NBSs) on seasonal soil functioning. Extracellular enzymes are the primary drivers of soil organic matter breakdown and assessing their activity and ecological stoichiometry can serve as an indicator of microbial nutrient demand and status. The primary goal of this study was to assess the nutrient status of the soil microbial biomass in olive orchards as affected by non-tillage (NT), the addition of pruning residues (PR), the combination of pruning residues and legumes (PL), the addition of pruning residues with no-tillage (PNT), and biochar (BI) relative to conventional tillage (TI). For this, we measured microbial C, N, and P acquisition through activities of key extracellular enzymes, 1,4-b-glucosidase (BG), 1,4-ß-N-acetylglucosaminidase (NAG), and acid/alkaline phosphatases (AP) in olive orchards.  Soil samples were taken in four comparable olive orchards in Crete, Greece, over six seasons (autumn 2022 to spring 2024). The proportional activity of C vs. N acquiring enzymes (BG/ [BG + NAG]) was analyzed relative to C and  P acquiring enzyme activity  (BG/[BG + AP]). We then calculated the vector length (quantifying the relative C vs. nutrient limitation) and angle (quantifying the relative P vs. N limitation).

Our preliminary analyses reveal significant seasonal and treatment-specific variations in microbial nutrient status and cycling. During Autumn 2022 and Winter 2023, a strong positive correlation between the C:N ratio and vector length indicates that microbes prioritize C-mineralizing enzymes (BG), likely due to slower decomposition rates and limited organic C availability under cooler conditions. This supports the idea that microbial communities focus on C acquisition under C-limiting conditions during the off-season. In Spring 2023, the highest BG/(BG+AP) ratios were observed indicating a shift toward P acquisition, likely driven by increased plant P demand during active plant and microbial growth. BI-treated soils showed higher BG/(BG+NAG) and BG/(BG+AP) ratios, lower AP activity (compared to TI), and larger vector angles, indicating increased P acquisition, and suggesting that biochar alleviates P limitation, especially in spring. Increased BG/(BG+AP) ratios in the presence of legumes (PL), particularly in spring, suggest that organic N from legumes helped the microbes to prioritize P during plant growth peaks. Larger vector angles in spring further indicated that PL enhanced microbial P acquisition during high-demand periods.

Seasonal shifts in microbial nutrient stoichiometry (biomass C:N:P), shifting enzyme activities, and changes in soil chemistry illustrate that nature-based solution (NBS) treatments such as BI and RP can alleviate microbial nutrient constraints and promote balanced nutrient cycling, thus providing viable tools for restoration of degraded orchard soils.

Keywords: Ecological stoichiometry, soil enzyme activity, soil organic matter, b-1,4-glucosidase, b-1,4-N-acetylglucosaminidase, phosphatase.