Biochar, Manure, and Sawdust Alter Long-Term Water Retention Dynamics in Degraded Soil

Biochars are porous but more recalcitrant that non-pyrolyzed organic materials, hence when biochar is added to soil it may cause more persistent alterations to soil water dynamics as compare to other materials. In this 6-yr, outdoor, large-pot study, we amended an irrigated calcareous silt loam with a single, one-time, 1 or 2% dry wt. (22.4, 44.8 Mg ha-1) application of either hardwood biochar, manure, sawdust, or acidified sawdust, a combined 1% biochar+2% manure treatment, and an untreated control. Each year the soils were hand-tilled and planted to a local crop. Soil water retention and plant available water (PAW, g H2O per g dry soil, being that retained between potentials of -10 to -1500kPa) were measured on spring soil samples. Across all years, the combined 1% biochar+2% manure produced the greatest PAW (0.262), with PAW in all treatments following the order of: 1% biochar+2% manure > all 2% rates > all 1% rates > control (0.222). In most years, the 2% treatments increased soil PAW relative to the control, i.e. their PAW ratios exceeded unity. The PAW ratios for 2% rates varied with amendment and year (P<0.0001). PAW ratios for: i) 2% manure peaked in yr 1 and declined to a minimum in yr 3; ii) 2% biochar, 2% sawdust, and 2% acid sawdust were least in yr 1 and peaked between yr 3 and 5; and iii) 1% biochar+2% manure was consistently near a maximum during the study period. Biochar effects on PAW were no more persistent than that of non-pyrolyzed amendments during the 6-yr study. Several factors influenced soil PAW development in addition to the quantity of the added highly porous amendment. Results suggest that water repellent properties and particle size distribution of the added organic material substantially influenced water retention dynamics in amended soils.