Carbon Dynamics in Management Intensive Grazing Dairy Systems.

In our first year of this project we have focused on (a) designing and modifying equipment to quantify enteric methane and CO₂fluxes and determine changes in manure composition as a function of forage species and maturity (Obj. 1 and 2) and quantifying baseline carbon stocks and fluxes across our Management Intensive Grazing Dairy (MiGD) chronosequence (Obj. 3).

(a) Gas flux and manure composition as function of forage species and maturity

Monitoring methane production from fermentation of forage in our simulated rumen reactors is a key objective of this project that required modification of our existing fermentors. After exploring commercially available biogas analyzers we designed a custom system to interface with our fermentors using component parts purchased from various vendors and in-house design expertise. Assembly of the system is underway to provide continuous monitoring of methane, carbon dioxide, and oxygen concentrations along with a central flow meter to calculate daily gas production from each of the 8 fermenters slated for this project. The system construction is expected to be complete by the end of April, 2012 and we will begin fermentation of forage samples in May, 2012. Toward this end we have harvested Bermuda grass across a maturity gradient of ranging from 7 d to 48 d of regrowth, and it is prepared for introduction to our fermentors. In addition, we have designed and purchased a growth chamber and gas flow system to produce 13C labeled forages for use in our fermentation experiments and subsequent soil incubations with labeled manure from these forages.

(b) Soil Carbon Stocks and fluxes across the MiGD Chronosequence

 

Total soil carbon and nitrogen concentrations, particle size and natural abundance stable isotopes (¹³C, ¹⁵N) have been measured in 1m deep representative soil pits and in composite cores of the top 5, 15 and 30cm at each of the sites across the MiGD chronosequence. The above ground biomass forage species and root biomass were sampled for stable isotope determination. Carbon concentration and composition measurements were separated into different depths and measured on bulk and clay-particle size fractions. A suite of carbon compositional methods have been developed and utilized including 13C-NMR, pyrolysis GC-MS, and a lignin extraction method. A subset of samples have been utilized to examine mineral assemblage using X-ray diffraction and selective extractions. We have begun a carbon lability experiment using 30% hydrogen peroxide and microbial incubations on both bulk and clay-particle size fractions. Total C and N as well as stable isotopes (¹³C, ¹⁵N) were measured throughout the experiment. Carbon compositional changes were also tracked at the beginning and end of the incubation using pyrolysis GC-MS.

Field carbon dioxide flux measurements have been conducted monthly at each chronosequence site beginning in October 2011 using a Licor 6400XT and PVC collars placed in the field. Carbon compositional measurements described above will be utilized to determine the degree of soil organic matter decomposition at each depth and within each particle size fraction.