Publications
Short-term soil carbon dynamics of humic fractions in low-input and organic cropping systems
Timothy A. Doane, Olivier C. Devevre, William R. Horwath
Observing changes in soil organic matter (SOM) is a fundamental part of defining the carbon
cycle in natural and cultivated environments. However, relying on changes in the mass of soil C over
short periods often produces conflicting results because of errors associated with sampling and
analysis. In addition, C mass balance studies provide little interpretation of processes or turnover of
specific C fractions. In the following study, we used C isotope and chemical separation of soil
organic C to observe short-term soil C dynamics. With corn as the source of tracer C in two cover
crop-based agricultural systems, natural abundance 13C measurements were used to identify changes
in soil humic fractions (humic acid, fulvic acid, and humin) during two seasons under organic or
low-input management treatments. All three fractions showed significant accumulation or turnover
of C, with the fulvic acid fraction showing the most frequent but the smallest changes. The fulvic
acid fraction showed a 5–9% turnover of C compared to 16% C turnover in the humic acid fraction.
The stable soil C fraction defined as humin also exhibited an 8% turnover of C. The different humic
fractions were affected at different times in the two treatments, supporting the idea that individual
humic fractions may have different roles in C cycling depending on inputs and seasonal conditions.
Modelling the turnover of 15N-labelled fertilizer and cover crop in soil and it's recovery by maize
A. Hadas, T. A. Doane, A. W. Kramer, C. Van Kessel & W. R. Horwath
Combining fertilizer and organic inputs to synchronize N supply in alternative cropping systems in California
Andrew W. Kramer, Timothy A. Doane, William R. Horwath, Chris van Kessel
One of the principal aims of alternative cropping systems is to minimize excessive loss of N while maximizing N use
efficiency and meeting cropNrequirements. Many such cropping systems substitute intensive application of synthetic fertilizer
with organic inputs, such as N2-fixing legumes. The effectiveness of legume residues as a N source for subsequent crops
depends heavily on temporal N release from the residue during the growing season. A field experiment with 15N-labeled
fertilizer and 15N-labeled vetch residue was conducted to determine the temporal pattern of N release from both sources in
conventional and alternative cropping systems in California. The experiment was conducted within conventional (fertilizer),
low-input (fertilizer and organic N), and organic (organic N only) cropping systems established 9 year previously. Availability
ofNfrom the labeled inputswas determined based on uptake bymaize (Zea mays L.). Uptake of totalNand 15Nbymaize in each
cropping system was monitored at 10 day intervals from 50 to 90 days after seeding for determination of uptake rates. Uptake
of N from fertilizer in the conventional system was greater than uptake of N from vetch in the low-input and organic systems.
Uptake of N from vetch was delayed, but with a sustained availability later in the season. Uptake rates of N from fertilizer
peaked at 4.3 kgNha-1 per day between 70 and 80 days while those from vetch residue reached a maximum of 0.6 kgNha-1
per day during the same time period. Grain and N yield at harvest did not differ between cropping systems despite different
temporal and quantitative availability of N from organic and inorganic N inputs. This demonstrates that optimum yields can be
achieved under management which uses alternative sources of N and can successfully match N availability with crop uptake.
A comparison of soil quality indexing methods for vegetable production systems in Northern California
S.S. Andrews, D.L. Karlen, J.P. Mitchell
Consultants, farm advisors, resource conservationists, and other land managers may benefit from decision tools that help
identify the most sustainable management practices. Indices of soil quality (SQIs) can provide this service. Various methods
were tested for choosing a minimum data set (MDS), transforming the indicators, and calculating indices using data from
alternative vegetable production systems being evaluated near Davis, California. The MDS components were chosen using
expert opinion (EO) or principal components analysis (PCA) as a data reduction technique. Multiple regressions of the MDS
indicators (as independent variables) against indicators representing management goals (as iterative dependent variables)
showed no significant differences between the EO and PCA selection techniques in their abilities to explain variability within
each sustainable management goal. Linear and non-linear scoring techniques were also compared for MDS indicators. The
non-linear scoring method was determined to be more representative of system function than the linear method. Finally,
indicator scores were combined using either an additive index, a weighted additive index, or a decision support system. For
almost all indexing combinations, the organic system received significantly higher SQI values than the lowinput or conventional
treatments. The efficacy of the indices was tested by comparisons with individual indicators, variables representative of
management goals, and another multivariate technique for decision making that used all available data rather than a subset
(MDS). Comparison with the comprehensive multivariate technique showed results similar to all of the indexing combinations
except the additive and weighted indices using the linearly scored, EO-selected MDS. This suggests that a small number of
carefully chosen soil quality indicators, when used in a simple, non-linearly scored index, can adequately provide information
needed for selection of best management practices.
Annual dynamics of soil organic matter in the context of long-term trends
Timothy A. Doane and William R. Horwath
Long-term research has provided a great deal of information regarding the influence
of management on the equilibrium dynamics of soil organic matter (SOM), although
short-term dynamics remain largely uninvestigated. An improved approach to
characterizing SOM dynamics in managed ecosystems would consider both short-term
and long-term changes in content and composition. This approach and its implications are
illustrated for an experimental site comparing agricultural management practices.
Changes in soil C composition were assessed semiquantitatively using 13C natural
abundance measurements, demonstrating their useful although rarely applied role in
short-term studies. This information is a valuable complement to long-term data, since net
differences since the site’s inception fail to reveal a timeline marked by repeated changes
in soil C content and composition. Such data are also useful for reinforcing and
understanding long-term simulation models, which are typically driven by temporally
dynamic events but are often fit against temporally sparse SOM data sets.