was primarily due to increased cash balances, continued purchases of investment securities, and strong organic growth in the loan portfolio, offset partially by the forgiveness of PPP loans. Average interest bearing liabilities increased $326.8 million, or 18.6%, to $2.09 billion for the year ended December 31, 2021, from $1.76 billion for the year ended December 31, 2020. The increase in average interest bearing liabilities was primarily due to an increase in interest bearing deposits and the issuance of subordinated debentures in the second quarter of 2021, partially offset by a decrease in notes payable and FHLB advances.
Average interest earning assets produced a fully tax-equivalent yield of 4.16% for the year ended December 31, 2021, compared to 4.51% for the year ended December 31, 2020. The decline in the yield on interest earning assets was primarily due to excess cash balances and the historically low interest rate environment resulting in lower loan and security yields. The average rate paid on interest bearing liabilities was 0.93% for the year ended December 31, 2021, compared to 1.53% for the year ended December 31, 2020 primarily due to lower rates paid on deposits, the payoff of the Company’s notes payable and the early extinguishment of $94.0 million of higher priced FHLB term advances, offset partially by strong growth of interest bearing deposits and the issuance of additional subordinated debentures.
Interest Income. Total interest income on a tax-equivalent basis was $129.7 million for the year ended December 31, 2021, compared to $115.7 million for the year ended December 31, 2020. The $14.0 million, or 12.1%, increase in total interest income on a tax-equivalent basis was primarily due to continued organic growth in the loan portfolio, as well as PPP loan income.
Interest income on cash investments increased $30,000, or 17.4%, for the year ended December 31, 2021, compared to the year ended December 31, 2020, primarily due to a $52.1 million, or 65.0%, increase in average cash balances, due to strong deposit inflows. Interest income on the investment securities portfolio on a fully-tax equivalent basis increased $738,000, or 7.7%, for the year ended December 31, 2021, compared to the year ended December 31, 2020, primarily due to a $70.8 million, or 22.0%, increase in average balances between the two periods, which was partially offset by a 34 basis point decline in the aggregate portfolio yield, driven by the historically low interest rate environment.
Interest income on loans, on a fully-tax equivalent basis, for the year ended December 31, 2021 was $119.0 million, compared to $105.6 million for the year ended December 31, 2020. The $13.4 million, or 12.7%, increase was due to a $430.4 million, or 20.0%, increase in the average balance of loans outstanding from continued organic loan growth, which was partially offset by a 30 basis point decline in the average yield on loans. The aggregate loan yield, excluding PPP loans decreased to 4.54% for the year ended December 31, 2021, which was 45 basis points lower than 4.99% for the year ended December 31, 2020, due to the historically low interest rate environment.
Interest Expense. Interest expense on interest bearing liabilities decreased $7.5 million, or 27.9%, to $19.4 million for the year ended December 31, 2021, compared to $26.9 million for the year ended December 31, 2020. The cost of interest bearing liabilities declined 60 basis points to 0.93% for the year ended December 31, 2021, compared to 1.53% for the year ended December 31, 2020. The decline was primarily due to lower rates paid on deposits, and the early extinguishment of $94.0 million of higher priced FHLB term advances, offset partially by growth of interest bearing deposits and the issuance of additional subordinated debentures.
Interest expense on deposits decreased to $13.8 million for the year ended December 31, 2021, compared to $19.8 million for the year ended December 31, 2020. The $6.0 million, or 30.1%, decrease in interest expense on deposits was primarily due to deposit rate cuts consistent with a lower rate environment and the repricing of time deposits, partially offset by the average balance of interest bearing deposits increasing by $404.4 million, or 26.3%. The cost of total deposits declined 42 basis points from 0.93% for the year ended December 31, 2020, to 0.51% for the year ended December 31, 2021.
Interest expense on borrowings decreased $1.5 million to $5.5 million for the year ended December 31, 2021, compared to $7.0 million for the year ended December 31, 2020. This decrease was primarily due to the lower average balance of federal funds purchased, the payoff of the Company’s note payable, the early extinguishment of $94.0 million of higher priced FHLB term advances, and the partial early redemption of $11.3 million of subordinated debentures