Modeling Diurnal Fluctuations in Water Content of Surface-Applied Broiler Litter.

Typical application rates of broiler litter to grasslands range from 3,000 to 10,000 kg ha^-1 which results in a 2-4 mm depth of litter on the surface of the pasture primed to interact with both atmospheric and soil conditions.  Broiler litter at typical water content (WC) has a very low water potential (WP) (-30 MPa at 0.25 g H₂O g^-1 on a dry-weight basis), which under high relative humidity (RH) results in a large WP gradient between litter and atmosphere. Changes in RH near the soil surface may lead to fluctuating litter WC, nitrogen mineralization, and subsequent ammonia volatilization.  To accurately model diurnal fluctuations in litter WC as a function of atmospheric WP, both laboratory and field experiments measuring litter WP as a function of RH were performed.  The water release curve of the broiler litter was determined and at field capacity (approx. 2.5 g H₂O g^-1) litter WP was -5 MPa and 0.01 g  H₂O g^-1  WP was equivalent to -185 MPa.  A 5 d study in a dynamic flow through system with diurnal fluctuations of RH (from 0.3 to 0.9) and temperature (from 6 to 25°C) litter WP ranged from -30 MPa to -180 MPa.  While in a 72 h field study, litter WP fluctuated from -13 MPa to -230 MPa.  Using the relationship between RH and litter WP, diurnal fluctuations in litter WC were accurately modeled as a function of litter and air WP.