Scenario-Based Analysis

Soil Carbon and Annual Nitrous-Oxide Results, 2024–2045

The scenario-based analysis compares four scenarios through December 2045. The reference uses business-as-usual management, CESM2 climate, and SSP370. Each alternative changes one dimension relative to the reference.

Table 1: Scenarios used in the 2024–2045 analysis.

Scenario	Management	Climate model	Pathway	Purpose
Reference	BAU	CESM2	SSP370	Reference scenario
Management	NBS	CESM2	SSP370	Management comparison
Climate model	BAU	MPI-ESM1-2-HR	SSP370	Climate-model sensitivity
Pathway	BAU	CESM2	SSP585	Emissions-pathway sensitivity

Soil-carbon context

Table 2: Statewide soil-carbon stocks and changes, Tg C. The inventory and scenario values come from separate simulation periods and are shown as context rather than as a continuous time series.

Measure	Annual crops	Woody perennial crops	Total
Inventory: December 2023 stock	113.22	87.93	201.15
Inventory: January 2016 to December 2023 change	-18.66	-18.50	-37.16
Reference scenario: January 2024 stock	124.96	95.84	220.80
Reference scenario: January 2024 to December 2045 change	-30.52	-14.81	-45.33

The negative reference-scenario change may include adjustment from a non-stable initial state. Scenario effects are therefore evaluated primarily as matched differences from the reference trajectory.

Scenario effects relative to the reference

Table 3: Soil-carbon change relative to the reference, Tg C. Contrasts preserve original ensemble-member identity before summarization. Positive values indicate more soil carbon relative to the reference trajectory.

Contrast	Annual crops	Woody perennial crops	Statewide mean	Statewide p05–p95
Management minus reference	0.73	0.83	1.55	-1.81 to 3.86
Climate model minus reference	0.36	-0.77	-0.41	-5.06 to 3.84
Pathway minus reference	-1.85	-1.87	-3.71	-6.16 to -1.31

Management retains an average of 1.55 Tg C more than the reference trajectory by December 2045. Changing the climate model produces little change in the statewide mean but different annual- and woody-crop responses. SSP585 reduces soil carbon by an additional 3.71 Tg C relative to the reference.

Figure 1: County soil-carbon change relative to the reference scenario. The three panels share a signed color scale.

Scenario soil-carbon responses

Figure 2: Statewide soil-carbon change and matched scenario contrasts. Panel A connects each original ensemble member’s reference change to its change under management, the alternate climate model, and the alternate pathway; boxes show the median and interquartile range. Panel B shows the corresponding within-member differences on a tighter scale. Positive values in Panel B indicate more soil carbon relative to the reference trajectory, not necessarily an absolute stock gain.

Table 4: Statewide December 2045 minus January 2024 soil-carbon stock, Tg C.

Scenario	Mean	p05	p95
Reference	-45.33	-98.26	17.87
Management	-43.77	-97.21	20.86
Climate model	-45.74	-95.55	15.98
Pathway	-49.04	-101.24	11.96

Annual N₂O-N

Figure 3: Annual N₂O-N for the reference and management scenarios. Lines show ensemble means; ribbons show empirical member p05–p95.

Scenario N₂O-N is integrated over each calendar year and reported in kg N ha^-1 yr^-1. The current annual comparison covers the reference and management scenarios; it is an interim two-scenario result rather than the finalized four-scenario annual-gas product.

Figure 4: Management-minus-reference annual N₂O-N. Differences pair the same ensemble-member IDs across the two separately fitted downscaling models.

Table 5: Selected annual N₂O-N means, kg N ha^-1 yr^-1.

Year	Reference	Management	Management minus reference
2024	23.71	23.85	0.14
2030	22.12	22.46	0.34
2035	21.55	22.34	0.79
2040	15.06	15.55	0.48
2045	20.95	22.06	1.11

The management mean is higher than the reference in most years, although the same-member range commonly crosses zero. Methane is not shown here; methane analysis will focus on crop fields with at least one rice season.

Related practice-change benchmark

A separate practice-change benchmark compares paired treatment and reference simulations at statewide design points with published synthesis estimates. Cover crops, reduced tillage, zero tillage, and organic amendment increased modeled soil carbon. The cover-crop and tillage effects were roughly half the synthesis estimates, while the organic-amendment effect was about one quarter. The design includes annual- and woody-crop systems, but uses one fixed parameterization for 2016–2023 and is distinct from both the inventory and the 2024–2045 ensemble scenario analysis reported here.